=====================

Model field reference

=====================

.. module:: django.db.models.fields

   :synopsis: Built-in field types.

.. currentmodule:: django.db.models

This document contains all the API references of :class:`Field` including the

`field options`_ and `field types`_ Django offers.

.. seealso::

    If the built-in fields don't do the trick, you can try `django-localflavor

    <https://github.com/django/django-localflavor>`_ (`documentation

    <https://django-localflavor.readthedocs.io/>`_), which contains assorted

    pieces of code that are useful for particular countries and cultures.

    Also, you can easily :doc:`write your own custom model fields

    </howto/custom-model-fields>`.

.. note::

    Technically, these models are defined in :mod:`django.db.models.fields`, but

    for convenience they're imported into :mod:`django.db.models`; the standard

    convention is to use ``from django.db import models`` and refer to fields as

    ``models.<Foo>Field``.

.. _common-model-field-options:

Field options

=============

The following arguments are available to all field types. All are optional.

``null``

--------

.. attribute:: Field.null

If ``True``, Django will store empty values as ``NULL`` in the database. Default

is ``False``.

Avoid using :attr:`~Field.null` on string-based fields such as

:class:`CharField` and :class:`TextField`. If a string-based field has

``null=True``, that means it has two possible values for "no data": ``NULL``,

and the empty string. In most cases, it's redundant to have two possible values

for "no data;" the Django convention is to use the empty string, not

``NULL``. One exception is when a :class:`CharField` has both ``unique=True``

and ``blank=True`` set. In this situation, ``null=True`` is required to avoid

unique constraint violations when saving multiple objects with blank values.

For both string-based and non-string-based fields, you will also need to

set ``blank=True`` if you wish to permit empty values in forms, as the

:attr:`~Field.null` parameter only affects database storage

(see :attr:`~Field.blank`).

.. note::

    When using the Oracle database backend, the value ``NULL`` will be stored to

    denote the empty string regardless of this attribute.

``blank``

---------

.. attribute:: Field.blank

If ``True``, the field is allowed to be blank. Default is ``False``.

Note that this is different than :attr:`~Field.null`. :attr:`~Field.null` is

purely database-related, whereas :attr:`~Field.blank` is validation-related. If

a field has ``blank=True``, form validation will allow entry of an empty value.

If a field has ``blank=False``, the field will be required.

.. admonition:: Supplying missing values

    ``blank=True`` can be used with fields having ``null=False``, but this will

    require implementing :meth:`~django.db.models.Model.clean` on the model in

    order to programmatically supply any missing values.

.. _field-choices:

``choices``

-----------

.. attribute:: Field.choices

A :term:`sequence` consisting itself of iterables of exactly two items (e.g.

``[(A, B), (A, B) ...]``) to use as choices for this field. If choices are

given, they're enforced by :ref:`model validation <validating-objects>` and the

default form widget will be a select box with these choices instead of the

standard text field.

The first element in each tuple is the actual value to be set on the model,

and the second element is the human-readable name. For example::

    YEAR_IN_SCHOOL_CHOICES = [

        ('FR', 'Freshman'),

        ('SO', 'Sophomore'),

        ('JR', 'Junior'),

        ('SR', 'Senior'),

        ('GR', 'Graduate'),

    ]

Generally, it's best to define choices inside a model class, and to

define a suitably-named constant for each value::

    from django.db import models

    class Student(models.Model):

        FRESHMAN = 'FR'

        SOPHOMORE = 'SO'

        JUNIOR = 'JR'

        SENIOR = 'SR'

        GRADUATE = 'GR'

        YEAR_IN_SCHOOL_CHOICES = [

            (FRESHMAN, 'Freshman'),

            (SOPHOMORE, 'Sophomore'),

            (JUNIOR, 'Junior'),

            (SENIOR, 'Senior'),

            (GRADUATE, 'Graduate'),

        ]

        year_in_school = models.CharField(

            max_length=2,

            choices=YEAR_IN_SCHOOL_CHOICES,

            default=FRESHMAN,

        )

        def is_upperclass(self):

            return self.year_in_school in {self.JUNIOR, self.SENIOR}

Though you can define a choices list outside of a model class and then

refer to it, defining the choices and names for each choice inside the

model class keeps all of that information with the class that uses it,

and helps reference the choices (e.g, ``Student.SOPHOMORE``

will work anywhere that the ``Student`` model has been imported).

.. _field-choices-named-groups:

You can also collect your available choices into named groups that can

be used for organizational purposes::

    MEDIA_CHOICES = [

        ('Audio', (

                ('vinyl', 'Vinyl'),

                ('cd', 'CD'),

            )

        ),

        ('Video', (

                ('vhs', 'VHS Tape'),

                ('dvd', 'DVD'),

            )

        ),

        ('unknown', 'Unknown'),

    ]

The first element in each tuple is the name to apply to the group. The

second element is an iterable of 2-tuples, with each 2-tuple containing

a value and a human-readable name for an option. Grouped options may be

combined with ungrouped options within a single list (such as the

``'unknown'`` option in this example).

For each model field that has :attr:`~Field.choices` set, Django will add a

method to retrieve the human-readable name for the field's current value. See

:meth:`~django.db.models.Model.get_FOO_display` in the database API

documentation.

Note that choices can be any sequence object -- not necessarily a list or

tuple. This lets you construct choices dynamically. But if you find yourself

hacking :attr:`~Field.choices` to be dynamic, you're probably better off using

a proper database table with a :class:`ForeignKey`. :attr:`~Field.choices` is

meant for static data that doesn't change much, if ever.

.. note::

    A new migration is created each time the order of ``choices`` changes.

.. _field-choices-blank-label:

Unless :attr:`blank=False<Field.blank>` is set on the field along with a

:attr:`~Field.default` then a label containing ``"---------"`` will be rendered

with the select box. To override this behavior, add a tuple to ``choices``

containing ``None``; e.g. ``(None, 'Your String For Display')``.

Alternatively, you can use an empty string instead of ``None`` where this makes

sense - such as on a :class:`~django.db.models.CharField`.

.. _field-choices-enum-types:

Enumeration types

~~~~~~~~~~~~~~~~~

In addition, Django provides enumeration types that you can subclass to define

choices in a concise way::

    from django.utils.translation import gettext_lazy as _

    class Student(models.Model):

        class YearInSchool(models.TextChoices):

            FRESHMAN = 'FR', _('Freshman')

            SOPHOMORE = 'SO', _('Sophomore')

            JUNIOR = 'JR', _('Junior')

            SENIOR = 'SR', _('Senior')

            GRADUATE = 'GR', _('Graduate')

        year_in_school = models.CharField(

            max_length=2,

            choices=YearInSchool.choices,

            default=YearInSchool.FRESHMAN,

        )

        def is_upperclass(self):

            return self.year_in_school in {

                self.YearInSchool.JUNIOR,

                self.YearInSchool.SENIOR,

            }

These work similar to :mod:`enum` from Python's standard library, but with some

modifications:

* Enum member values are a tuple of arguments to use when constructing the

  concrete data type. Django supports adding an extra string value to the end

  of this tuple to be used as the human-readable name, or ``label``. The

  ``label`` can be a lazy translatable string. Thus, in most cases, the member

  value will be a ``(value, label)`` two-tuple. See below for :ref:`an example

  of subclassing choices <field-choices-enum-subclassing>` using a more complex

  data type. If a tuple is not provided, or the last item is not a (lazy)

  string, the ``label`` is :ref:`automatically generated

  <field-choices-enum-auto-label>` from the member name.

* A ``.label`` property is added on values, to return the human-readable name.

* A number of custom properties are added to the enumeration classes --

  ``.choices``, ``.labels``, ``.values``, and ``.names`` -- to make it easier

  to access lists of those separate parts of the enumeration. Use ``.choices``

  as a suitable value to pass to :attr:`~Field.choices` in a field definition.

  .. warning::

    These property names cannot be used as member names as they would conflict.

* The use of :func:`enum.unique()` is enforced to ensure that values cannot be

  defined multiple times. This is unlikely to be expected in choices for a

  field.

Note that using ``YearInSchool.SENIOR``, ``YearInSchool['SENIOR']``, or

``YearInSchool('SR')`` to access or lookup enum members work as expected, as do

the ``.name`` and ``.value`` properties on the members.

.. _field-choices-enum-auto-label:

If you don't need to have the human-readable names translated, you can have

them inferred from the member name (replacing underscores with spaces and using

title-case)::

    >>> class Vehicle(models.TextChoices):

    ...     CAR = 'C'

    ...     TRUCK = 'T'

    ...     JET_SKI = 'J'

    ...

    >>> Vehicle.JET_SKI.label

    'Jet Ski'

Since the case where the enum values need to be integers is extremely common,

Django provides an ``IntegerChoices`` class. For example::

    class Card(models.Model):

        class Suit(models.IntegerChoices):

            DIAMOND = 1

            SPADE = 2

            HEART = 3

            CLUB = 4

        suit = models.IntegerField(choices=Suit.choices)

It is also possible to make use of the `Enum Functional API

<https://docs.python.org/3/library/enum.html#functional-api>`_ with the caveat

that labels are automatically generated as highlighted above::

    >>> MedalType = models.TextChoices('MedalType', 'GOLD SILVER BRONZE')

    >>> MedalType.choices

    [('GOLD', 'Gold'), ('SILVER', 'Silver'), ('BRONZE', 'Bronze')]

    >>> Place = models.IntegerChoices('Place', 'FIRST SECOND THIRD')

    >>> Place.choices

    [(1, 'First'), (2, 'Second'), (3, 'Third')]

.. _field-choices-enum-subclassing:

If you require support for a concrete data type other than ``int`` or ``str``,

you can subclass ``Choices`` and the required concrete data type, e.g.

:class:`~datetime.date` for use with :class:`~django.db.models.DateField`::

    class MoonLandings(datetime.date, models.Choices):

        APOLLO_11 = 1969, 7, 20, 'Apollo 11 (Eagle)'

        APOLLO_12 = 1969, 11, 19, 'Apollo 12 (Intrepid)'

        APOLLO_14 = 1971, 2, 5, 'Apollo 14 (Antares)'

        APOLLO_15 = 1971, 7, 30, 'Apollo 15 (Falcon)'

        APOLLO_16 = 1972, 4, 21, 'Apollo 16 (Orion)'

        APOLLO_17 = 1972, 12, 11, 'Apollo 17 (Challenger)'

There are some additional caveats to be aware of:

- Enumeration types do not support :ref:`named groups

  <field-choices-named-groups>`.

- Because an enumeration with a concrete data type requires all values to match

  the type, overriding the :ref:`blank label <field-choices-blank-label>`

  cannot be achieved by creating a member with a value of ``None``. Instead,

  set the ``__empty__`` attribute on the class::

    class Answer(models.IntegerChoices):

        NO = 0, _('No')

        YES = 1, _('Yes')

        __empty__ = _('(Unknown)')

``db_column``

-------------

.. attribute:: Field.db_column

The name of the database column to use for this field. If this isn't given,

Django will use the field's name.

If your database column name is an SQL reserved word, or contains

characters that aren't allowed in Python variable names -- notably, the

hyphen -- that's OK. Django quotes column and table names behind the

scenes.

``db_index``

------------

.. attribute:: Field.db_index

If ``True``, a database index will be created for this field.

``db_tablespace``

-----------------

.. attribute:: Field.db_tablespace

The name of the :doc:`database tablespace </topics/db/tablespaces>` to use for

this field's index, if this field is indexed. The default is the project's

:setting:`DEFAULT_INDEX_TABLESPACE` setting, if set, or the

:attr:`~Options.db_tablespace` of the model, if any. If the backend doesn't

support tablespaces for indexes, this option is ignored.

``default``

-----------

.. attribute:: Field.default

The default value for the field. This can be a value or a callable object. If

callable it will be called every time a new object is created.

The default can't be a mutable object (model instance, ``list``, ``set``, etc.),

as a reference to the same instance of that object would be used as the default

value in all new model instances. Instead, wrap the desired default in a

callable. For example, if you want to specify a default ``dict`` for

:class:`~django.db.models.JSONField`, use a function::

    def contact_default():

        return {"email": "[email protected]"}

    contact_info = JSONField("ContactInfo", default=contact_default)

``lambda``\s can't be used for field options like ``default`` because they

can't be :ref:`serialized by migrations <migration-serializing>`. See that

documentation for other caveats.

For fields like :class:`ForeignKey` that map to model instances, defaults

should be the value of the field they reference (``pk`` unless

:attr:`~ForeignKey.to_field` is set) instead of model instances.

The default value is used when new model instances are created and a value

isn't provided for the field. When the field is a primary key, the default is

also used when the field is set to ``None``.

``editable``

------------

.. attribute:: Field.editable

If ``False``, the field will not be displayed in the admin or any other

:class:`~django.forms.ModelForm`. They are also skipped during :ref:`model

validation <validating-objects>`. Default is ``True``.

``error_messages``

------------------

.. attribute:: Field.error_messages

The ``error_messages`` argument lets you override the default messages that the

field will raise. Pass in a dictionary with keys matching the error messages you

want to override.

Error message keys include ``null``, ``blank``, ``invalid``, ``invalid_choice``,

``unique``, and ``unique_for_date``. Additional error message keys are

specified for each field in the `Field types`_ section below.

These error messages often don't propagate to forms. See

:ref:`considerations-regarding-model-errormessages`.

``help_text``

-------------

.. attribute:: Field.help_text

Extra "help" text to be displayed with the form widget. It's useful for

documentation even if your field isn't used on a form.

Note that this value is *not* HTML-escaped in automatically-generated

forms. This lets you include HTML in :attr:`~Field.help_text` if you so

desire. For example::

    help_text="Please use the following format: <em>YYYY-MM-DD</em>."

Alternatively you can use plain text and

:func:`django.utils.html.escape` to escape any HTML special characters. Ensure

that you escape any help text that may come from untrusted users to avoid a

cross-site scripting attack.

``primary_key``

---------------

.. attribute:: Field.primary_key

If ``True``, this field is the primary key for the model.

If you don't specify ``primary_key=True`` for any field in your model, Django

will automatically add a field to hold the primary key, so you don't need to

set ``primary_key=True`` on any of your fields unless you want to override the

default primary-key behavior. The type of auto-created primary key fields can

be specified per app in :attr:`AppConfig.default_auto_field

<django.apps.AppConfig.default_auto_field>` or globally in the

:setting:`DEFAULT_AUTO_FIELD` setting. For more, see

:ref:`automatic-primary-key-fields`.

``primary_key=True`` implies :attr:`null=False <Field.null>` and

:attr:`unique=True <Field.unique>`. Only one primary key is allowed on an

object.

The primary key field is read-only. If you change the value of the primary

key on an existing object and then save it, a new object will be created

alongside the old one.

The primary key field is set to ``None`` when

:meth:`deleting <django.db.models.Model.delete>` an object.

``unique``

----------

.. attribute:: Field.unique

If ``True``, this field must be unique throughout the table.

This is enforced at the database level and by model validation. If

you try to save a model with a duplicate value in a :attr:`~Field.unique`

field, a :exc:`django.db.IntegrityError` will be raised by the model's

:meth:`~django.db.models.Model.save` method.

This option is valid on all field types except :class:`ManyToManyField` and

:class:`OneToOneField`.

Note that when ``unique`` is ``True``, you don't need to specify

:attr:`~Field.db_index`, because ``unique`` implies the creation of an index.

``unique_for_date``

-------------------

.. attribute:: Field.unique_for_date

Set this to the name of a :class:`DateField` or :class:`DateTimeField` to

require that this field be unique for the value of the date field.

For example, if you have a field ``title`` that has

``unique_for_date="pub_date"``, then Django wouldn't allow the entry of two

records with the same ``title`` and ``pub_date``.

Note that if you set this to point to a :class:`DateTimeField`, only the date

portion of the field will be considered. Besides, when :setting:`USE_TZ` is

``True``, the check will be performed in the :ref:`current time zone

<default-current-time-zone>` at the time the object gets saved.

This is enforced by :meth:`Model.validate_unique()` during model validation

but not at the database level. If any :attr:`~Field.unique_for_date` constraint

involves fields that are not part of a :class:`~django.forms.ModelForm` (for

example, if one of the fields is listed in ``exclude`` or has

:attr:`editable=False<Field.editable>`), :meth:`Model.validate_unique()` will

skip validation for that particular constraint.

``unique_for_month``

--------------------

.. attribute:: Field.unique_for_month

Like :attr:`~Field.unique_for_date`, but requires the field to be unique with

respect to the month.

``unique_for_year``

-------------------

.. attribute:: Field.unique_for_year

Like :attr:`~Field.unique_for_date` and :attr:`~Field.unique_for_month`.

``verbose_name``

----------------

.. attribute:: Field.verbose_name

A human-readable name for the field. If the verbose name isn't given, Django

will automatically create it using the field's attribute name, converting

underscores to spaces. See :ref:`Verbose field names <verbose-field-names>`.

``validators``

--------------

.. attribute:: Field.validators

A list of validators to run for this field. See the :doc:`validators

documentation </ref/validators>` for more information.

.. _model-field-types:

Field types

===========

.. currentmodule:: django.db.models

``AutoField``

-------------

.. class:: AutoField(**options)

An :class:`IntegerField` that automatically increments

according to available IDs. You usually won't need to use this directly; a

primary key field will automatically be added to your model if you don't specify

otherwise. See :ref:`automatic-primary-key-fields`.

``BigAutoField``

----------------

.. class:: BigAutoField(**options)

A 64-bit integer, much like an :class:`AutoField` except that it is

guaranteed to fit numbers from ``1`` to ``9223372036854775807``.

``BigIntegerField``

-------------------

.. class:: BigIntegerField(**options)

A 64-bit integer, much like an :class:`IntegerField` except that it is

guaranteed to fit numbers from ``-9223372036854775808`` to

``9223372036854775807``. The default form widget for this field is a

:class:`~django.forms.NumberInput`.

``BinaryField``

---------------

.. class:: BinaryField(max_length=None, **options)

A field to store raw binary data. It can be assigned :class:`bytes`,

:class:`bytearray`, or :class:`memoryview`.

By default, ``BinaryField`` sets :attr:`~Field.editable` to ``False``, in which

case it can't be included in a :class:`~django.forms.ModelForm`.

.. attribute:: BinaryField.max_length

    Optional. The maximum length (in bytes) of the field. The maximum length is

    enforced in Django's validation using

    :class:`~django.core.validators.MaxLengthValidator`.

.. admonition:: Abusing ``BinaryField``

    Although you might think about storing files in the database, consider that

    it is bad design in 99% of the cases. This field is *not* a replacement for

    proper :doc:`static files </howto/static-files/index>` handling.

``BooleanField``

----------------

.. class:: BooleanField(**options)

A true/false field.

The default form widget for this field is :class:`~django.forms.CheckboxInput`,

or :class:`~django.forms.NullBooleanSelect` if :attr:`null=True <Field.null>`.

The default value of ``BooleanField`` is ``None`` when :attr:`Field.default`

isn't defined.

``CharField``

-------------

.. class:: CharField(max_length=None, **options)

A string field, for small- to large-sized strings.

For large amounts of text, use :class:`~django.db.models.TextField`.

The default form widget for this field is a :class:`~django.forms.TextInput`.

:class:`CharField` has the following extra arguments:

.. attribute:: CharField.max_length

    Required. The maximum length (in characters) of the field. The max_length

    is enforced at the database level and in Django's validation using

    :class:`~django.core.validators.MaxLengthValidator`.

    .. note::

        If you are writing an application that must be portable to multiple

        database backends, you should be aware that there are restrictions on

        ``max_length`` for some backends. Refer to the :doc:`database backend

        notes </ref/databases>` for details.

.. attribute:: CharField.db_collation

    Optional. The database collation name of the field.

    .. note::

        Collation names are not standardized. As such, this will not be

        portable across multiple database backends.

    .. admonition:: Oracle

        Oracle supports collations only when the ``MAX_STRING_SIZE`` database

        initialization parameter is set to ``EXTENDED``.

``DateField``

-------------

.. class:: DateField(auto_now=False, auto_now_add=False, **options)

A date, represented in Python by a ``datetime.date`` instance. Has a few extra,

optional arguments:

.. attribute:: DateField.auto_now

    Automatically set the field to now every time the object is saved. Useful

    for "last-modified" timestamps. Note that the current date is *always*

    used; it's not just a default value that you can override.

    The field is only automatically updated when calling :meth:`Model.save()

    <django.db.models.Model.save>`. The field isn't updated when making updates

    to other fields in other ways such as :meth:`QuerySet.update()

    <django.db.models.query.QuerySet.update>`, though you can specify a custom

    value for the field in an update like that.

.. attribute:: DateField.auto_now_add

    Automatically set the field to now when the object is first created. Useful

    for creation of timestamps. Note that the current date is *always* used;

    it's not just a default value that you can override. So even if you

    set a value for this field when creating the object, it will be ignored.

    If you want to be able to modify this field, set the following instead of

    ``auto_now_add=True``:

    * For :class:`DateField`: ``default=date.today`` - from

      :meth:`datetime.date.today`

    * For :class:`DateTimeField`: ``default=timezone.now`` - from

      :func:`django.utils.timezone.now`

The default form widget for this field is a

:class:`~django.forms.DateInput`. The admin adds a JavaScript calendar,

and a shortcut for "Today". Includes an additional ``invalid_date`` error

message key.

The options ``auto_now_add``, ``auto_now``, and ``default`` are mutually exclusive.

Any combination of these options will result in an error.

.. note::

    As currently implemented, setting ``auto_now`` or ``auto_now_add`` to

    ``True`` will cause the field to have ``editable=False`` and ``blank=True``

    set.

.. note::

    The ``auto_now`` and ``auto_now_add`` options will always use the date in

    the :ref:`default timezone <default-current-time-zone>` at the moment of

    creation or update. If you need something different, you may want to

    consider using your own callable default or overriding ``save()`` instead

    of using ``auto_now`` or ``auto_now_add``; or using a ``DateTimeField``

    instead of a ``DateField`` and deciding how to handle the conversion from

    datetime to date at display time.

``DateTimeField``

-----------------

.. class:: DateTimeField(auto_now=False, auto_now_add=False, **options)

A date and time, represented in Python by a ``datetime.datetime`` instance.

Takes the same extra arguments as :class:`DateField`.

The default form widget for this field is a single

:class:`~django.forms.DateTimeInput`. The admin uses two separate

:class:`~django.forms.TextInput` widgets with JavaScript shortcuts.

``DecimalField``

----------------

.. class:: DecimalField(max_digits=None, decimal_places=None, **options)

A fixed-precision decimal number, represented in Python by a

:class:`~decimal.Decimal` instance. It validates the input using

:class:`~django.core.validators.DecimalValidator`.

Has the following **required** arguments:

.. attribute:: DecimalField.max_digits

    The maximum number of digits allowed in the number. Note that this number

    must be greater than or equal to ``decimal_places``.

.. attribute:: DecimalField.decimal_places

    The number of decimal places to store with the number.

For example, to store numbers up to ``999.99`` with a resolution of 2 decimal

places, you'd use::

    models.DecimalField(..., max_digits=5, decimal_places=2)

And to store numbers up to approximately one billion with a resolution of 10

decimal places::

    models.DecimalField(..., max_digits=19, decimal_places=10)

The default form widget for this field is a :class:`~django.forms.NumberInput`

when :attr:`~django.forms.Field.localize` is ``False`` or

:class:`~django.forms.TextInput` otherwise.

.. note::

    For more information about the differences between the

    :class:`FloatField` and :class:`DecimalField` classes, please

    see :ref:`FloatField vs. DecimalField <floatfield_vs_decimalfield>`. You

    should also be aware of :ref:`SQLite limitations <sqlite-decimal-handling>`

    of decimal fields.

``DurationField``

-----------------

.. class:: DurationField(**options)

A field for storing periods of time - modeled in Python by

:class:`~python:datetime.timedelta`. When used on PostgreSQL, the data type

used is an ``interval`` and on Oracle the data type is ``INTERVAL DAY(9) TO

SECOND(6)``. Otherwise a ``bigint`` of microseconds is used.

.. note::

    Arithmetic with ``DurationField`` works in most cases. However on all

    databases other than PostgreSQL, comparing the value of a ``DurationField``

    to arithmetic on ``DateTimeField`` instances will not work as expected.

``EmailField``

--------------

.. class:: EmailField(max_length=254, **options)

A :class:`CharField` that checks that the value is a valid email address using

:class:`~django.core.validators.EmailValidator`.

``FileField``

-------------

.. class:: FileField(upload_to='', storage=None, max_length=100, **options)

A file-upload field.

.. note::

    The ``primary_key`` argument isn't supported and will raise an error if

    used.

Has the following optional arguments:

.. attribute:: FileField.upload_to

    This attribute provides a way of setting the upload directory and file name,

    and can be set in two ways. In both cases, the value is passed to the

    :meth:`Storage.save() <django.core.files.storage.Storage.save>` method.

    If you specify a string value or a :class:`~pathlib.Path`, it may contain

    :func:`~time.strftime` formatting, which will be replaced by the date/time

    of the file upload (so that uploaded files don't fill up the given

    directory). For example::

        class MyModel(models.Model):

            # file will be uploaded to MEDIA_ROOT/uploads

            upload = models.FileField(upload_to='uploads/')

            # or...

            # file will be saved to MEDIA_ROOT/uploads/2015/01/30

            upload = models.FileField(upload_to='uploads/%Y/%m/%d/')

    If you are using the default

    :class:`~django.core.files.storage.FileSystemStorage`, the string value

    will be appended to your :setting:`MEDIA_ROOT` path to form the location on

    the local filesystem where uploaded files will be stored. If you are using

    a different storage, check that storage's documentation to see how it

    handles ``upload_to``.

    ``upload_to`` may also be a callable, such as a function. This will be

    called to obtain the upload path, including the filename. This callable must

    accept two arguments and return a Unix-style path (with forward slashes)

    to be passed along to the storage system. The two arguments are:

    ======================  ===============================================

    Argument                Description

    ======================  ===============================================

    ``instance``            An instance of the model where the

                            ``FileField`` is defined. More specifically,

                            this is the particular instance where the

                            current file is being attached.

                            In most cases, this object will not have been

                            saved to the database yet, so if it uses the

                            default ``AutoField``, *it might not yet have a

                            value for its primary key field*.

    ``filename``            The filename that was originally given to the

                            file. This may or may not be taken into account

                            when determining the final destination path.

    ======================  ===============================================

    For example::

        def user_directory_path(instance, filename):

            # file will be uploaded to MEDIA_ROOT/user_<id>/<filename>

            return 'user_{0}/{1}'.format(instance.user.id, filename)

        class MyModel(models.Model):

            upload = models.FileField(upload_to=user_directory_path)

.. attribute:: FileField.storage

    A storage object, or a callable which returns a storage object. This

    handles the storage and retrieval of your files. See :doc:`/topics/files`

    for details on how to provide this object.

The default form widget for this field is a

:class:`~django.forms.ClearableFileInput`.

Using a :class:`FileField` or an :class:`ImageField` (see below) in a model

takes a few steps:

#. In your settings file, you'll need to define :setting:`MEDIA_ROOT` as the

   full path to a directory where you'd like Django to store uploaded files.

   (For performance, these files are not stored in the database.) Define

   :setting:`MEDIA_URL` as the base public URL of that directory. Make sure

   that this directory is writable by the web server's user account.

#. Add the :class:`FileField` or :class:`ImageField` to your model, defining

   the :attr:`~FileField.upload_to` option to specify a subdirectory of

   :setting:`MEDIA_ROOT` to use for uploaded files.

#. All that will be stored in your database is a path to the file

   (relative to :setting:`MEDIA_ROOT`). You'll most likely want to use the

   convenience :attr:`~django.db.models.fields.files.FieldFile.url` attribute

   provided by Django. For example, if your :class:`ImageField` is called

   ``mug_shot``, you can get the absolute path to your image in a template with

   ``{{ object.mug_shot.url }}``.

For example, say your :setting:`MEDIA_ROOT` is set to ``'/home/media'``, and

:attr:`~FileField.upload_to` is set to ``'photos/%Y/%m/%d'``. The ``'%Y/%m/%d'``

part of :attr:`~FileField.upload_to` is :func:`~time.strftime` formatting;

``'%Y'`` is the four-digit year, ``'%m'`` is the two-digit month and ``'%d'`` is

the two-digit day. If you upload a file on Jan. 15, 2007, it will be saved in

the directory ``/home/media/photos/2007/01/15``.

If you wanted to retrieve the uploaded file's on-disk filename, or the file's

size, you could use the :attr:`~django.core.files.File.name` and

:attr:`~django.core.files.File.size` attributes respectively; for more

information on the available attributes and methods, see the

:class:`~django.core.files.File` class reference and the :doc:`/topics/files`

topic guide.

.. note::

    The file is saved as part of saving the model in the database, so the actual

    file name used on disk cannot be relied on until after the model has been

    saved.

The uploaded file's relative URL can be obtained using the

:attr:`~django.db.models.fields.files.FieldFile.url` attribute. Internally,

this calls the :meth:`~django.core.files.storage.Storage.url` method of the

underlying :class:`~django.core.files.storage.Storage` class.

.. _file-upload-security:

Note that whenever you deal with uploaded files, you should pay close attention

to where you're uploading them and what type of files they are, to avoid

security holes. *Validate all uploaded files* so that you're sure the files are

what you think they are. For example, if you blindly let somebody upload files,

without validation, to a directory that's within your web server's document

root, then somebody could upload a CGI or PHP script and execute that script by

visiting its URL on your site. Don't allow that.

Also note that even an uploaded HTML file, since it can be executed by the

browser (though not by the server), can pose security threats that are

equivalent to XSS or CSRF attacks.

:class:`FileField` instances are created in your database as ``varchar``

columns with a default max length of 100 characters. As with other fields, you

can change the maximum length using the :attr:`~CharField.max_length` argument.

``FileField`` and ``FieldFile``

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. currentmodule:: django.db.models.fields.files

.. class:: FieldFile

When you access a :class:`~django.db.models.FileField` on a model, you are

given an instance of :class:`FieldFile` as a proxy for accessing the underlying

file.

The API of :class:`FieldFile` mirrors that of :class:`~django.core.files.File`,

with one key difference: *The object wrapped by the class is not necessarily a

wrapper around Python's built-in file object.* Instead, it is a wrapper around

the result of the :attr:`Storage.open()<django.core.files.storage.Storage.open>`

method, which may be a :class:`~django.core.files.File` object, or it may be a

custom storage's implementation of the :class:`~django.core.files.File` API.

In addition to the API inherited from :class:`~django.core.files.File` such as

``read()`` and ``write()``, :class:`FieldFile` includes several methods that

can be used to interact with the underlying file:

.. warning::

    Two methods of this class, :meth:`~FieldFile.save` and

    :meth:`~FieldFile.delete`, default to saving the model object of the

    associated ``FieldFile`` in the database.

.. attribute:: FieldFile.name

The name of the file including the relative path from the root of the

:class:`~django.core.files.storage.Storage` of the associated

:class:`~django.db.models.FileField`.

.. attribute:: FieldFile.path

A read-only property to access the file's local filesystem path by calling the

:meth:`~django.core.files.storage.Storage.path` method of the underlying

:class:`~django.core.files.storage.Storage` class.

.. attribute:: FieldFile.size

The result of the underlying :attr:`Storage.size()

<django.core.files.storage.Storage.size>` method.

.. attribute:: FieldFile.url

A read-only property to access the file's relative URL by calling the

:meth:`~django.core.files.storage.Storage.url` method of the underlying

:class:`~django.core.files.storage.Storage` class.

.. method:: FieldFile.open(mode='rb')

Opens or reopens the file associated with this instance in the specified

``mode``. Unlike the standard Python ``open()`` method, it doesn't return a

file descriptor.

Since the underlying file is opened implicitly when accessing it, it may be

unnecessary to call this method except to reset the pointer to the underlying

file or to change the ``mode``.

.. method:: FieldFile.close()

Behaves like the standard Python ``file.close()`` method and closes the file

associated with this instance.

.. method:: FieldFile.save(name, content, save=True)

This method takes a filename and file contents and passes them to the storage

class for the field, then associates the stored file with the model field.

If you want to manually associate file data with

:class:`~django.db.models.FileField` instances on your model, the ``save()``

method is used to persist that file data.

Takes two required arguments: ``name`` which is the name of the file, and

``content`` which is an object containing the file's contents.  The

optional ``save`` argument controls whether or not the model instance is

saved after the file associated with this field has been altered. Defaults to

``True``.

Note that the ``content`` argument should be an instance of

:class:`django.core.files.File`, not Python's built-in file object.

You can construct a :class:`~django.core.files.File` from an existing

Python file object like this::

    from django.core.files import File

    # Open an existing file using Python's built-in open()

    f = open('/path/to/hello.world')

    myfile = File(f)

Or you can construct one from a Python string like this::

    from django.core.files.base import ContentFile

    myfile = ContentFile("hello world")

For more information, see :doc:`/topics/files`.

.. method:: FieldFile.delete(save=True)

Deletes the file associated with this instance and clears all attributes on

the field. Note: This method will close the file if it happens to be open when

``delete()`` is called.

The optional ``save`` argument controls whether or not the model instance is

saved after the file associated with this field has been deleted. Defaults to

``True``.

Note that when a model is deleted, related files are not deleted. If you need

to cleanup orphaned files, you'll need to handle it yourself (for instance,

with a custom management command that can be run manually or scheduled to run

periodically via e.g. cron).

.. currentmodule:: django.db.models

``FilePathField``

-----------------

.. class:: FilePathField(path='', match=None, recursive=False, allow_files=True, allow_folders=False, max_length=100, **options)

A :class:`CharField` whose choices are limited to the filenames in a certain

directory on the filesystem. Has some special arguments, of which the first is

**required**:

.. attribute:: FilePathField.path

    Required. The absolute filesystem path to a directory from which this

    :class:`FilePathField` should get its choices. Example: ``"/home/images"``.

    ``path`` may also be a callable, such as a function to dynamically set the

    path at runtime. Example::

        import os

        from django.conf import settings

        from django.db import models

        def images_path():

            return os.path.join(settings.LOCAL_FILE_DIR, 'images')

        class MyModel(models.Model):

            file = models.FilePathField(path=images_path)

.. attribute:: FilePathField.match

    Optional. A regular expression, as a string, that :class:`FilePathField`

    will use to filter filenames. Note that the regex will be applied to the

    base filename, not the full path. Example: ``"foo.*\.txt$"``, which will

    match a file called ``foo23.txt`` but not ``bar.txt`` or ``foo23.png``.

.. attribute:: FilePathField.recursive

    Optional. Either ``True`` or ``False``. Default is ``False``. Specifies

    whether all subdirectories of :attr:`~FilePathField.path` should be included

.. attribute:: FilePathField.allow_files

    Optional.  Either ``True`` or ``False``.  Default is ``True``.  Specifies

    whether files in the specified location should be included.  Either this or

    :attr:`~FilePathField.allow_folders` must be ``True``.

.. attribute:: FilePathField.allow_folders

    Optional.  Either ``True`` or ``False``.  Default is ``False``.  Specifies

    whether folders in the specified location should be included.  Either this

    or :attr:`~FilePathField.allow_files` must be ``True``.

The one potential gotcha is that :attr:`~FilePathField.match` applies to the

base filename, not the full path. So, this example::

    FilePathField(path="/home/images", match="foo.*", recursive=True)

...will match ``/home/images/foo.png`` but not ``/home/images/foo/bar.png``

because the :attr:`~FilePathField.match` applies to the base filename

(``foo.png`` and ``bar.png``).

:class:`FilePathField` instances are created in your database as ``varchar``

columns with a default max length of 100 characters. As with other fields, you

can change the maximum length using the :attr:`~CharField.max_length` argument.

``FloatField``

--------------

.. class:: FloatField(**options)

A floating-point number represented in Python by a ``float`` instance.

The default form widget for this field is a :class:`~django.forms.NumberInput`

when :attr:`~django.forms.Field.localize` is ``False`` or

:class:`~django.forms.TextInput` otherwise.

.. _floatfield_vs_decimalfield:

.. admonition:: ``FloatField`` vs. ``DecimalField``

    The :class:`FloatField` class is sometimes mixed up with the

    :class:`DecimalField` class. Although they both represent real numbers, they

    represent those numbers differently. ``FloatField`` uses Python's ``float``

    type internally, while ``DecimalField`` uses Python's ``Decimal`` type. For

    information on the difference between the two, see Python's documentation

    for the :mod:`decimal` module.

``GenericIPAddressField``

-------------------------

.. class:: GenericIPAddressField(protocol='both', unpack_ipv4=False, **options)

An IPv4 or IPv6 address, in string format (e.g. ``192.0.2.30`` or

``2a02:42fe::4``). The default form widget for this field is a

:class:`~django.forms.TextInput`.

The IPv6 address normalization follows :rfc:`4291#section-2.2` section 2.2,

including using the IPv4 format suggested in paragraph 3 of that section, like

``::ffff:192.0.2.0``. For example, ``2001:0::0:01`` would be normalized to

``2001::1``, and ``::ffff:0a0a:0a0a`` to ``::ffff:10.10.10.10``. All characters

are converted to lowercase.

.. attribute:: GenericIPAddressField.protocol

    Limits valid inputs to the specified protocol.

    Accepted values are ``'both'`` (default), ``'IPv4'``

    or ``'IPv6'``. Matching is case insensitive.

.. attribute:: GenericIPAddressField.unpack_ipv4

    Unpacks IPv4 mapped addresses like ``::ffff:192.0.2.1``.

    If this option is enabled that address would be unpacked to

    ``192.0.2.1``. Default is disabled. Can only be used

    when ``protocol`` is set to ``'both'``.

If you allow for blank values, you have to allow for null values since blank

values are stored as null.

``ImageField``

--------------

.. class:: ImageField(upload_to=None, height_field=None, width_field=None, max_length=100, **options)

Inherits all attributes and methods from :class:`FileField`, but also

validates that the uploaded object is a valid image.

In addition to the special attributes that are available for :class:`FileField`,

an :class:`ImageField` also has ``height`` and ``width`` attributes.

To facilitate querying on those attributes, :class:`ImageField` has the

following optional arguments:

.. attribute:: ImageField.height_field

    Name of a model field which will be auto-populated with the height of the

    image each time the model instance is saved.

.. attribute:: ImageField.width_field

    Name of a model field which will be auto-populated with the width of the

    image each time the model instance is saved.

Requires the `Pillow`_ library.

.. _Pillow: https://pillow.readthedocs.io/en/latest/

:class:`ImageField` instances are created in your database as ``varchar``

columns with a default max length of 100 characters. As with other fields, you

can change the maximum length using the :attr:`~CharField.max_length` argument.

The default form widget for this field is a

:class:`~django.forms.ClearableFileInput`.

``IntegerField``

----------------

.. class:: IntegerField(**options)

An integer. Values from ``-2147483648`` to ``2147483647`` are safe in all

databases supported by Django.

It uses :class:`~django.core.validators.MinValueValidator` and

:class:`~django.core.validators.MaxValueValidator` to validate the input based

on the values that the default database supports.

The default form widget for this field is a :class:`~django.forms.NumberInput`

when :attr:`~django.forms.Field.localize` is ``False`` or

:class:`~django.forms.TextInput` otherwise.

``JSONField``

-------------

.. class:: JSONField(encoder=None, decoder=None, **options)

A field for storing JSON encoded data. In Python the data is represented in its

Python native format: dictionaries, lists, strings, numbers, booleans and

``None``.

``JSONField`` is supported on MariaDB, MySQL 5.7.8+, Oracle, PostgreSQL, and

SQLite (with the :ref:`JSON1 extension enabled <sqlite-json1>`).

.. attribute:: JSONField.encoder

    An optional :py:class:`json.JSONEncoder` subclass to serialize data types

    not supported by the standard JSON serializer (e.g. ``datetime.datetime``

    or :class:`~python:uuid.UUID`). For example, you can use the

    :class:`~django.core.serializers.json.DjangoJSONEncoder` class.

    Defaults to ``json.JSONEncoder``.

.. attribute:: JSONField.decoder

    An optional :py:class:`json.JSONDecoder` subclass to deserialize the value

    retrieved from the database. The value will be in the format chosen by the

    custom encoder (most often a string). Your deserialization may need to

    account for the fact that you can't be certain of the input type. For

    example, you run the risk of returning a ``datetime`` that was actually a

    string that just happened to be in the same format chosen for

    ``datetime``\s.

    Defaults to ``json.JSONDecoder``.

If you give the field a :attr:`~django.db.models.Field.default`, ensure it's an

immutable object, such as a ``str``, or a callable object that returns a fresh

mutable object each time, such as ``dict`` or a function. Providing a mutable

default object like ``default={}`` or ``default=[]`` shares the one object

between all model instances.

To query ``JSONField`` in the database, see :ref:`querying-jsonfield`.

.. admonition:: Indexing

    :class:`~django.db.models.Index` and :attr:`.Field.db_index` both create a

    B-tree index, which isn't particularly helpful when querying ``JSONField``.

    On PostgreSQL only, you can use

    :class:`~django.contrib.postgres.indexes.GinIndex` that is better suited.

.. admonition:: PostgreSQL users

    PostgreSQL has two native JSON based data types: ``json`` and ``jsonb``.

    The main difference between them is how they are stored and how they can be

    queried. PostgreSQL's ``json`` field is stored as the original string

    representation of the JSON and must be decoded on the fly when queried

    based on keys. The ``jsonb`` field is stored based on the actual structure

    of the JSON which allows indexing. The trade-off is a small additional cost

    on writing to the ``jsonb`` field. ``JSONField`` uses ``jsonb``.

.. admonition:: Oracle users

    Oracle Database does not support storing JSON scalar values. Only JSON

    objects and arrays (represented in Python using :py:class:`dict` and

    :py:class:`list`) are supported.

``PositiveBigIntegerField``

---------------------------

.. class:: PositiveBigIntegerField(**options)

Like a :class:`PositiveIntegerField`, but only allows values under a certain

(database-dependent) point. Values from ``0`` to ``9223372036854775807`` are

safe in all databases supported by Django.

``PositiveIntegerField``

------------------------

.. class:: PositiveIntegerField(**options)

Like an :class:`IntegerField`, but must be either positive or zero (``0``).

Values from ``0`` to ``2147483647`` are safe in all databases supported by

Django. The value ``0`` is accepted for backward compatibility reasons.

``PositiveSmallIntegerField``

-----------------------------

.. class:: PositiveSmallIntegerField(**options)

Like a :class:`PositiveIntegerField`, but only allows values under a certain

(database-dependent) point. Values from ``0`` to ``32767`` are safe in all

databases supported by Django.

``SlugField``

-------------

.. class:: SlugField(max_length=50, **options)

:term:`Slug <slug>` is a newspaper term. A slug is a short label for something,

containing only letters, numbers, underscores or hyphens. They're generally used

in URLs.

Like a CharField, you can specify :attr:`~CharField.max_length` (read the note

about database portability and :attr:`~CharField.max_length` in that section,

too). If :attr:`~CharField.max_length` is not specified, Django will use a

default length of 50.

Implies setting :attr:`Field.db_index` to ``True``.

It is often useful to automatically prepopulate a SlugField based on the value

of some other value.  You can do this automatically in the admin using

:attr:`~django.contrib.admin.ModelAdmin.prepopulated_fields`.

It uses :class:`~django.core.validators.validate_slug` or

:class:`~django.core.validators.validate_unicode_slug` for validation.

.. attribute:: SlugField.allow_unicode

    If ``True``, the field accepts Unicode letters in addition to ASCII

    letters. Defaults to ``False``.

``SmallAutoField``

------------------

.. class:: SmallAutoField(**options)

Like an :class:`AutoField`, but only allows values under a certain

(database-dependent) limit. Values from ``1`` to ``32767`` are safe in all

databases supported by Django.

``SmallIntegerField``

---------------------

.. class:: SmallIntegerField(**options)

Like an :class:`IntegerField`, but only allows values under a certain

(database-dependent) point. Values from ``-32768`` to ``32767`` are safe in all

databases supported by Django.

``TextField``

-------------

.. class:: TextField(**options)

A large text field. The default form widget for this field is a

:class:`~django.forms.Textarea`.

If you specify a ``max_length`` attribute, it will be reflected in the

:class:`~django.forms.Textarea` widget of the auto-generated form field.

However it is not enforced at the model or database level. Use a

:class:`CharField` for that.

.. attribute:: TextField.db_collation

    Optional. The database collation name of the field.

    .. note::

        Collation names are not standardized. As such, this will not be

        portable across multiple database backends.

    .. admonition:: Oracle

        Oracle does not support collations for a ``TextField``.

``TimeField``

-------------

.. class:: TimeField(auto_now=False, auto_now_add=False, **options)

A time, represented in Python by a ``datetime.time`` instance. Accepts the same

auto-population options as :class:`DateField`.

The default form widget for this field is a :class:`~django.forms.TimeInput`.

The admin adds some JavaScript shortcuts.

``URLField``

------------

.. class:: URLField(max_length=200, **options)

A :class:`CharField` for a URL, validated by

:class:`~django.core.validators.URLValidator`.

The default form widget for this field is a :class:`~django.forms.URLInput`.

Like all :class:`CharField` subclasses, :class:`URLField` takes the optional

:attr:`~CharField.max_length` argument. If you don't specify

:attr:`~CharField.max_length`, a default of 200 is used.

``UUIDField``

-------------

.. class:: UUIDField(**options)

A field for storing universally unique identifiers. Uses Python's

:class:`~python:uuid.UUID` class. When used on PostgreSQL, this stores in a

``uuid`` datatype, otherwise in a ``char(32)``.

Universally unique identifiers are a good alternative to :class:`AutoField` for

:attr:`~Field.primary_key`. The database will not generate the UUID for you, so

it is recommended to use :attr:`~Field.default`::

    import uuid

    from django.db import models

    class MyUUIDModel(models.Model):

        id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)

        # other fields

Note that a callable (with the parentheses omitted) is passed to ``default``,

not an instance of ``UUID``.

.. admonition:: Lookups on PostgreSQL

    Using :lookup:`iexact`, :lookup:`contains`, :lookup:`icontains`,

    :lookup:`startswith`, :lookup:`istartswith`, :lookup:`endswith`, or

    :lookup:`iendswith` lookups on PostgreSQL don't work for values without

    hyphens, because PostgreSQL stores them in a hyphenated uuid datatype type.

Relationship fields

===================

.. module:: django.db.models.fields.related

   :synopsis: Related field types

.. currentmodule:: django.db.models

Django also defines a set of fields that represent relations.

.. _ref-foreignkey:

``ForeignKey``

--------------

.. class:: ForeignKey(to, on_delete, **options)

A many-to-one relationship. Requires two positional arguments: the class to

which the model is related and the :attr:`~ForeignKey.on_delete` option.

.. _recursive-relationships:

To create a recursive relationship -- an object that has a many-to-one

relationship with itself -- use ``models.ForeignKey('self',

on_delete=models.CASCADE)``.

.. _lazy-relationships:

If you need to create a relationship on a model that has not yet been defined,

you can use the name of the model, rather than the model object itself::

    from django.db import models

    class Car(models.Model):

        manufacturer = models.ForeignKey(

            'Manufacturer',

            on_delete=models.CASCADE,

        )

        # ...

    class Manufacturer(models.Model):

        # ...

        pass

Relationships defined this way on :ref:`abstract models

<abstract-base-classes>` are resolved when the model is subclassed as a

concrete model and are not relative to the abstract model's ``app_label``:

.. code-block:: python

    :caption: ``products/models.py``

    from django.db import models

    class AbstractCar(models.Model):

        manufacturer = models.ForeignKey('Manufacturer', on_delete=models.CASCADE)

        class Meta:

            abstract = True

.. code-block:: python

    :caption: ``production/models.py``

    from django.db import models

    from products.models import AbstractCar

    class Manufacturer(models.Model):

        pass

    class Car(AbstractCar):

        pass

    # Car.manufacturer will point to `production.Manufacturer` here.

To refer to models defined in another application, you can explicitly specify

a model with the full application label. For example, if the ``Manufacturer``

model above is defined in another application called ``production``, you'd

need to use::

    class Car(models.Model):

        manufacturer = models.ForeignKey(

            'production.Manufacturer',

            on_delete=models.CASCADE,

        )

This sort of reference, called a lazy relationship, can be useful when

resolving circular import dependencies between two applications.

A database index is automatically created on the ``ForeignKey``. You can

disable this by setting :attr:`~Field.db_index` to ``False``.  You may want to

avoid the overhead of an index if you are creating a foreign key for

consistency rather than joins, or if you will be creating an alternative index

like a partial or multiple column index.

Database Representation

~~~~~~~~~~~~~~~~~~~~~~~

Behind the scenes, Django appends ``"_id"`` to the field name to create its

database column name. In the above example, the database table for the ``Car``

model will have a ``manufacturer_id`` column. (You can change this explicitly by

specifying :attr:`~Field.db_column`) However, your code should never have to

deal with the database column name, unless you write custom SQL. You'll always

deal with the field names of your model object.

.. _foreign-key-arguments:

Arguments

~~~~~~~~~

:class:`ForeignKey` accepts other arguments that define the details of how the

relation works.

.. attribute:: ForeignKey.on_delete

    When an object referenced by a :class:`ForeignKey` is deleted, Django will

    emulate the behavior of the SQL constraint specified by the

    :attr:`on_delete` argument. For example, if you have a nullable

    :class:`ForeignKey` and you want it to be set null when the referenced

    object is deleted::

        user = models.ForeignKey(

            User,

            models.SET_NULL,

            blank=True,

            null=True,

        )

    ``on_delete`` doesn't create an SQL constraint in the database. Support for

    database-level cascade options :ticket:`may be implemented later <21961>`.

The possible values for :attr:`~ForeignKey.on_delete` are found in

:mod:`django.db.models`:

* .. attribute:: CASCADE

    Cascade deletes. Django emulates the behavior of the SQL constraint ON

    DELETE CASCADE and also deletes the object containing the ForeignKey.

    :meth:`.Model.delete` isn't called on related models, but the

    :data:`~django.db.models.signals.pre_delete` and

    :data:`~django.db.models.signals.post_delete` signals are sent for all

    deleted objects.

* .. attribute:: PROTECT

    Prevent deletion of the referenced object by raising

    :exc:`~django.db.models.ProtectedError`, a subclass of

    :exc:`django.db.IntegrityError`.

* .. attribute:: RESTRICT

    Prevent deletion of the referenced object by raising

    :exc:`~django.db.models.RestrictedError` (a subclass of

    :exc:`django.db.IntegrityError`). Unlike :attr:`PROTECT`, deletion of the

    referenced object is allowed if it also references a different object

    that is being deleted in the same operation, but via a :attr:`CASCADE`

    relationship.

    Consider this set of models::

        class Artist(models.Model):

            name = models.CharField(max_length=10)

        class Album(models.Model):

            artist = models.ForeignKey(Artist, on_delete=models.CASCADE)

        class Song(models.Model):

            artist = models.ForeignKey(Artist, on_delete=models.CASCADE)

            album = models.ForeignKey(Album, on_delete=models.RESTRICT)

    ``Artist`` can be deleted even if that implies deleting an ``Album``

    which is referenced by a ``Song``, because ``Song`` also references

    ``Artist`` itself through a cascading relationship. For example::

        >>> artist_one = Artist.objects.create(name='artist one')

        >>> artist_two = Artist.objects.create(name='artist two')

        >>> album_one = Album.objects.create(artist=artist_one)

        >>> album_two = Album.objects.create(artist=artist_two)

        >>> song_one = Song.objects.create(artist=artist_one, album=album_one)

        >>> song_two = Song.objects.create(artist=artist_one, album=album_two)

        >>> album_one.delete()

        # Raises RestrictedError.

        >>> artist_two.delete()

        # Raises RestrictedError.

        >>> artist_one.delete()

        (4, {'Song': 2, 'Album': 1, 'Artist': 1})

* .. attribute:: SET_NULL

    Set the :class:`ForeignKey` null; this is only possible if

    :attr:`~Field.null` is ``True``.

* .. attribute:: SET_DEFAULT

    Set the :class:`ForeignKey` to its default value; a default for the

    :class:`ForeignKey` must be set.

* .. function:: SET()

    Set the :class:`ForeignKey` to the value passed to

    :func:`~django.db.models.SET()`, or if a callable is passed in,

    the result of calling it. In most cases, passing a callable will be

    necessary to avoid executing queries at the time your ``models.py`` is

    imported::

        from django.conf import settings

        from django.contrib.auth import get_user_model

        from django.db import models

        def get_sentinel_user():

            return get_user_model().objects.get_or_create(username='deleted')[0]

        class MyModel(models.Model):

            user = models.ForeignKey(

                settings.AUTH_USER_MODEL,

                on_delete=models.SET(get_sentinel_user),

            )

* .. attribute:: DO_NOTHING

    Take no action. If your database backend enforces referential

    integrity, this will cause an :exc:`~django.db.IntegrityError` unless

    you manually add an SQL ``ON DELETE`` constraint to the database field.

.. attribute:: ForeignKey.limit_choices_to

    Sets a limit to the available choices for this field when this field is

    rendered using a ``ModelForm`` or the admin (by default, all objects

    in the queryset are available to choose). Either a dictionary, a

    :class:`~django.db.models.Q` object, or a callable returning a

    dictionary or :class:`~django.db.models.Q` object can be used.

    For example::

        staff_member = models.ForeignKey(

            User,

            on_delete=models.CASCADE,

            limit_choices_to={'is_staff': True},

        )

    causes the corresponding field on the ``ModelForm`` to list only ``Users``

    that have ``is_staff=True``. This may be helpful in the Django admin.

    The callable form can be helpful, for instance, when used in conjunction

    with the Python ``datetime`` module to limit selections by date range. For

    example::

        def limit_pub_date_choices():

            return {'pub_date__lte': datetime.date.today()}

        limit_choices_to = limit_pub_date_choices

    If ``limit_choices_to`` is or returns a :class:`Q object

    <django.db.models.Q>`, which is useful for :ref:`complex queries

    <complex-lookups-with-q>`, then it will only have an effect on the choices

    available in the admin when the field is not listed in

    :attr:`~django.contrib.admin.ModelAdmin.raw_id_fields` in the

    ``ModelAdmin`` for the model.

    .. note::

        If a callable is used for ``limit_choices_to``, it will be invoked

        every time a new form is instantiated. It may also be invoked when a

        model is validated, for example by management commands or the admin.

        The admin constructs querysets to validate its form inputs in various

        edge cases multiple times, so there is a possibility your callable may

        be invoked several times.

.. attribute:: ForeignKey.related_name

    The name to use for the relation from the related object back to this one.

    It's also the default value for :attr:`related_query_name` (the name to use

    for the reverse filter name from the target model). See the :ref:`related

    objects documentation <backwards-related-objects>` for a full explanation

    and example. Note that you must set this value when defining relations on

    :ref:`abstract models <abstract-base-classes>`; and when you do so

    :ref:`some special syntax <abstract-related-name>` is available.

    If you'd prefer Django not to create a backwards relation, set

    ``related_name`` to ``'+'`` or end it with ``'+'``. For example, this will

    ensure that the ``User`` model won't have a backwards relation to this

    model::

        user = models.ForeignKey(

            User,

            on_delete=models.CASCADE,

            related_name='+',

        )

.. attribute:: ForeignKey.related_query_name

    The name to use for the reverse filter name from the target model. It

    defaults to the value of :attr:`related_name` or

    :attr:`~django.db.models.Options.default_related_name` if set, otherwise it

    defaults to the name of the model::

        # Declare the ForeignKey with related_query_name

        class Tag(models.Model):

            article = models.ForeignKey(

                Article,

                on_delete=models.CASCADE,

                related_name="tags",

                related_query_name="tag",

            )

            name = models.CharField(max_length=255)

        # That's now the name of the reverse filter

        Article.objects.filter(tag__name="important")

    Like :attr:`related_name`, ``related_query_name`` supports app label and

    class interpolation via :ref:`some special syntax <abstract-related-name>`.

.. attribute:: ForeignKey.to_field

    The field on the related object that the relation is to. By default, Django

    uses the primary key of the related object. If you reference a different

    field, that field must have ``unique=True``.

.. attribute:: ForeignKey.db_constraint

    Controls whether or not a constraint should be created in the database for

    this foreign key. The default is ``True``, and that's almost certainly what

    you want; setting this to ``False`` can be very bad for data integrity.

    That said, here are some scenarios where you might want to do this:

    * You have legacy data that is not valid.

    * You're sharding your database.

    If this is set to ``False``, accessing a related object that doesn't exist

    will raise its ``DoesNotExist`` exception.

.. attribute:: ForeignKey.swappable

    Controls the migration framework's reaction if this :class:`ForeignKey`

    is pointing at a swappable model. If it is ``True`` - the default -

    then if the :class:`ForeignKey` is pointing at a model which matches

    the current value of ``settings.AUTH_USER_MODEL`` (or another swappable

    model setting) the relationship will be stored in the migration using

    a reference to the setting, not to the model directly.

    You only want to override this to be ``False`` if you are sure your

    model should always point toward the swapped-in model - for example,

    if it is a profile model designed specifically for your custom user model.

    Setting it to ``False`` does not mean you can reference a swappable model

    even if it is swapped out - ``False`` means that the migrations made

    with this ForeignKey will always reference the exact model you specify

    (so it will fail hard if the user tries to run with a User model you don't

    support, for example).

    If in doubt, leave it to its default of ``True``.

``ManyToManyField``

-------------------

.. class:: ManyToManyField(to, **options)

A many-to-many relationship. Requires a positional argument: the class to

which the model is related, which works exactly the same as it does for

:class:`ForeignKey`, including :ref:`recursive <recursive-relationships>` and

:ref:`lazy <lazy-relationships>` relationships.

Related objects can be added, removed, or created with the field's

:class:`~django.db.models.fields.related.RelatedManager`.

Database Representation

~~~~~~~~~~~~~~~~~~~~~~~

Behind the scenes, Django creates an intermediary join table to represent the

many-to-many relationship. By default, this table name is generated using the

name of the many-to-many field and the name of the table for the model that

contains it. Since some databases don't support table names above a certain

length, these table names will be automatically truncated and a uniqueness hash

will be used, e.g. ``author_books_9cdf``. You can manually provide the name of

the join table using the :attr:`~ManyToManyField.db_table` option.

.. _manytomany-arguments:

Arguments

~~~~~~~~~

:class:`ManyToManyField` accepts an extra set of arguments -- all optional --

that control how the relationship functions.

.. attribute:: ManyToManyField.related_name

    Same as :attr:`ForeignKey.related_name`.

.. attribute:: ManyToManyField.related_query_name

    Same as :attr:`ForeignKey.related_query_name`.

.. attribute:: ManyToManyField.limit_choices_to

    Same as :attr:`ForeignKey.limit_choices_to`.

.. attribute:: ManyToManyField.symmetrical

    Only used in the definition of ManyToManyFields on self. Consider the

    following model::

        from django.db import models

        class Person(models.Model):

            friends = models.ManyToManyField("self")

    When Django processes this model, it identifies that it has a

    :class:`ManyToManyField` on itself, and as a result, it doesn't add a

    ``person_set`` attribute to the ``Person`` class. Instead, the

    :class:`ManyToManyField` is assumed to be symmetrical -- that is, if I am

    your friend, then you are my friend.

    If you do not want symmetry in many-to-many relationships with ``self``, set

    :attr:`~ManyToManyField.symmetrical` to ``False``. This will force Django to

    add the descriptor for the reverse relationship, allowing

    :class:`ManyToManyField` relationships to be non-symmetrical.

.. attribute:: ManyToManyField.through

    Django will automatically generate a table to manage many-to-many

    relationships. However, if you want to manually specify the intermediary

    table, you can use the :attr:`~ManyToManyField.through` option to specify

    the Django model that represents the intermediate table that you want to

    use.

    The most common use for this option is when you want to associate

    :ref:`extra data with a many-to-many relationship

    <intermediary-manytomany>`.

    .. note::

        If you don't want multiple associations between the same instances, add

        a :class:`~django.db.models.UniqueConstraint` including the from and to

        fields. Django's automatically generated many-to-many tables include

        such a constraint.

    .. note::

        Recursive relationships using an intermediary model can't determine the

        reverse accessors names, as they would be the same. You need to set a

        :attr:`~ForeignKey.related_name` to at least one of them. If you'd

        prefer Django not to create a backwards relation, set ``related_name``

        to ``'+'``.

    If you don't specify an explicit ``through`` model, there is still an

    implicit ``through`` model class you can use to directly access the table

    created to hold the association. It has three fields to link the models.

    If the source and target models differ, the following fields are

    generated:

    * ``id``: the primary key of the relation.

    * ``<containing_model>_id``: the ``id`` of the model that declares the

      ``ManyToManyField``.

    * ``<other_model>_id``: the ``id`` of the model that the

      ``ManyToManyField`` points to.

    If the ``ManyToManyField`` points from and to the same model, the following

    fields are generated:

    * ``id``: the primary key of the relation.

    * ``from_<model>_id``: the ``id`` of the instance which points at the

      model (i.e. the source instance).

    * ``to_<model>_id``: the ``id`` of the instance to which the relationship

      points (i.e. the target model instance).

    This class can be used to query associated records for a given model

    instance like a normal model::

        Model.m2mfield.through.objects.all()

.. attribute:: ManyToManyField.through_fields

    Only used when a custom intermediary model is specified. Django will

    normally determine which fields of the intermediary model to use in order

    to establish a many-to-many relationship automatically. However,

    consider the following models::

        from django.db import models

        class Person(models.Model):

            name = models.CharField(max_length=50)

        class Group(models.Model):

            name = models.CharField(max_length=128)

            members = models.ManyToManyField(

                Person,

                through='Membership',

                through_fields=('group', 'person'),

            )

        class Membership(models.Model):

            group = models.ForeignKey(Group, on_delete=models.CASCADE)

            person = models.ForeignKey(Person, on_delete=models.CASCADE)

            inviter = models.ForeignKey(

                Person,

                on_delete=models.CASCADE,

                related_name="membership_invites",

            )

            invite_reason = models.CharField(max_length=64)

    ``Membership`` has *two* foreign keys to ``Person`` (``person`` and

    ``inviter``), which makes the relationship ambiguous and Django can't know

    which one to use. In this case, you must explicitly specify which

    foreign keys Django should use using ``through_fields``, as in the example

    above.

    ``through_fields`` accepts a 2-tuple ``('field1', 'field2')``, where

    ``field1`` is the name of the foreign key to the model the

    :class:`ManyToManyField` is defined on (``group`` in this case), and

    ``field2`` the name of the foreign key to the target model (``person``

    in this case).

    When you have more than one foreign key on an intermediary model to any

    (or even both) of the models participating in a many-to-many relationship,

    you *must* specify ``through_fields``. This also applies to

    :ref:`recursive relationships <recursive-relationships>`

    when an intermediary model is used and there are more than two

    foreign keys to the model, or you want to explicitly specify which two

    Django should use.

.. attribute:: ManyToManyField.db_table

    The name of the table to create for storing the many-to-many data. If this

    is not provided, Django will assume a default name based upon the names of:

    the table for the model defining the relationship and the name of the field

    itself.

.. attribute:: ManyToManyField.db_constraint

    Controls whether or not constraints should be created in the database for

    the foreign keys in the intermediary table. The default is ``True``, and

    that's almost certainly what you want; setting this to ``False`` can be

    very bad for data integrity. That said, here are some scenarios where you

    might want to do this:

    * You have legacy data that is not valid.

    * You're sharding your database.

    It is an error to pass both ``db_constraint`` and ``through``.

.. attribute:: ManyToManyField.swappable

    Controls the migration framework's reaction if this :class:`ManyToManyField`

    is pointing at a swappable model. If it is ``True`` - the default -

    then if the :class:`ManyToManyField` is pointing at a model which matches

    the current value of ``settings.AUTH_USER_MODEL`` (or another swappable

    model setting) the relationship will be stored in the migration using

    a reference to the setting, not to the model directly.

    You only want to override this to be ``False`` if you are sure your

    model should always point toward the swapped-in model - for example,

    if it is a profile model designed specifically for your custom user model.

    If in doubt, leave it to its default of ``True``.

:class:`ManyToManyField` does not support :attr:`~Field.validators`.

:attr:`~Field.null` has no effect since there is no way to require a

relationship at the database level.

``OneToOneField``

-----------------

.. class:: OneToOneField(to, on_delete, parent_link=False, **options)

A one-to-one relationship. Conceptually, this is similar to a

:class:`ForeignKey` with :attr:`unique=True <Field.unique>`, but the

"reverse" side of the relation will directly return a single object.

This is most useful as the primary key of a model which "extends"

another model in some way; :ref:`multi-table-inheritance` is

implemented by adding an implicit one-to-one relation from the child

model to the parent model, for example.

One positional argument is required: the class to which the model will be

related. This works exactly the same as it does for :class:`ForeignKey`,

including all the options regarding :ref:`recursive <recursive-relationships>`

and :ref:`lazy <lazy-relationships>` relationships.

If you do not specify the :attr:`~ForeignKey.related_name` argument for the

``OneToOneField``, Django will use the lowercase name of the current model as

default value.

With the following example::

    from django.conf import settings

    from django.db import models