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

Model instance reference

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

.. currentmodule:: django.db.models

This document describes the details of the ``Model`` API. It builds on the

material presented in the :doc:`model </topics/db/models>` and :doc:`database

query </topics/db/queries>` guides, so you'll probably want to read and

understand those documents before reading this one.

Throughout this reference we'll use the :ref:`example blog models

<queryset-model-example>` presented in the :doc:`database query guide

</topics/db/queries>`.

Creating objects

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

To create a new instance of a model, instantiate it like any other Python

class:

.. class:: Model(**kwargs)

The keyword arguments are the names of the fields you've defined on your model.

Note that instantiating a model in no way touches your database; for that, you

need to :meth:`~Model.save()`.

.. note::

    You may be tempted to customize the model by overriding the ``__init__``

    method. If you do so, however, take care not to change the calling

    signature as any change may prevent the model instance from being saved.

    Rather than overriding ``__init__``, try using one of these approaches:

    #. Add a classmethod on the model class::

        from django.db import models

        class Book(models.Model):

            title = models.CharField(max_length=100)

            @classmethod

            def create(cls, title):

                book = cls(title=title)

                # do something with the book

                return book

        book = Book.create("Pride and Prejudice")

    #. Add a method on a custom manager (usually preferred)::

        class BookManager(models.Manager):

            def create_book(self, title):

                book = self.create(title=title)

                # do something with the book

                return book

        class Book(models.Model):

            title = models.CharField(max_length=100)

            objects = BookManager()

        book = Book.objects.create_book("Pride and Prejudice")

Customizing model loading

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

.. classmethod:: Model.from_db(db, field_names, values)

The ``from_db()`` method can be used to customize model instance creation

when loading from the database.

The ``db`` argument contains the database alias for the database the model

is loaded from, ``field_names`` contains the names of all loaded fields, and

``values`` contains the loaded values for each field in ``field_names``. The

``field_names`` are in the same order as the ``values``. If all of the model's

fields are present, then ``values`` are guaranteed to be in the order

``__init__()`` expects them. That is, the instance can be created by

``cls(*values)``. If any fields are deferred, they won't appear in

``field_names``. In that case, assign a value of ``django.db.models.DEFERRED``

to each of the missing fields.

In addition to creating the new model, the ``from_db()`` method must set the

``adding`` and ``db`` flags in the new instance's :attr:`~Model._state` attribute.

Below is an example showing how to record the initial values of fields that

are loaded from the database::

    from django.db.models import DEFERRED

    @classmethod

    def from_db(cls, db, field_names, values):

        # Default implementation of from_db() (subject to change and could

        # be replaced with super()).

        if len(values) != len(cls._meta.concrete_fields):

            values = list(values)

            values.reverse()

            values = [

                values.pop() if f.attname in field_names else DEFERRED

                for f in cls._meta.concrete_fields

            ]

        instance = cls(*values)

        instance._state.adding = False

        instance._state.db = db

        # customization to store the original field values on the instance

        instance._loaded_values = dict(

            zip(field_names, (value for value in values if value is not DEFERRED))

        )

        return instance

    def save(self, *args, **kwargs):

        # Check how the current values differ from ._loaded_values. For example,

        # prevent changing the creator_id of the model. (This example doesn't

        # support cases where 'creator_id' is deferred).

        if not self._state.adding and (

                self.creator_id != self._loaded_values['creator_id']):

            raise ValueError("Updating the value of creator isn't allowed")

        super().save(*args, **kwargs)

The example above shows a full ``from_db()`` implementation to clarify how that

is done. In this case it would be possible to use a ``super()`` call in the

``from_db()`` method.

Refreshing objects from database

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

If you delete a field from a model instance, accessing it again reloads the

value from the database::

    >>> obj = MyModel.objects.first()

    >>> del obj.field

    >>> obj.field  # Loads the field from the database

.. method:: Model.refresh_from_db(using=None, fields=None)

If you need to reload a model's values from the database, you can use the

``refresh_from_db()`` method. When this method is called without arguments the

following is done:

#. All non-deferred fields of the model are updated to the values currently

   present in the database.

#. Any cached relations are cleared from the reloaded instance.

Only fields of the model are reloaded from the database. Other

database-dependent values such as annotations aren't reloaded. Any

:func:`@cached_property <django.utils.functional.cached_property>` attributes

aren't cleared either.

The reloading happens from the database the instance was loaded from, or from

the default database if the instance wasn't loaded from the database. The

``using`` argument can be used to force the database used for reloading.

It is possible to force the set of fields to be loaded by using the ``fields``

argument.

For example, to test that an ``update()`` call resulted in the expected

update, you could write a test similar to this::

    def test_update_result(self):

        obj = MyModel.objects.create(val=1)

        MyModel.objects.filter(pk=obj.pk).update(val=F('val') + 1)

        # At this point obj.val is still 1, but the value in the database

        # was updated to 2. The object's updated value needs to be reloaded

        # from the database.

        obj.refresh_from_db()

        self.assertEqual(obj.val, 2)

Note that when deferred fields are accessed, the loading of the deferred

field's value happens through this method. Thus it is possible to customize

the way deferred loading happens. The example below shows how one can reload

all of the instance's fields when a deferred field is reloaded::

    class ExampleModel(models.Model):

        def refresh_from_db(self, using=None, fields=None, **kwargs):

            # fields contains the name of the deferred field to be

            # loaded.

            if fields is not None:

                fields = set(fields)

                deferred_fields = self.get_deferred_fields()

                # If any deferred field is going to be loaded

                if fields.intersection(deferred_fields):

                    # then load all of them

                    fields = fields.union(deferred_fields)

            super().refresh_from_db(using, fields, **kwargs)

.. method:: Model.get_deferred_fields()

A helper method that returns a set containing the attribute names of all those

fields that are currently deferred for this model.

.. _validating-objects:

Validating objects

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

There are four steps involved in validating a model:

1. Validate the model fields - :meth:`Model.clean_fields()`

2. Validate the model as a whole - :meth:`Model.clean()`

3. Validate the field uniqueness - :meth:`Model.validate_unique()`

4. Validate the constraints - :meth:`Model.validate_constraints`

All four steps are performed when you call a model's :meth:`~Model.full_clean`

method.

When you use a :class:`~django.forms.ModelForm`, the call to

:meth:`~django.forms.Form.is_valid()` will perform these validation steps for

all the fields that are included on the form. See the :doc:`ModelForm

documentation </topics/forms/modelforms>` for more information. You should only

need to call a model's :meth:`~Model.full_clean()` method if you plan to handle

validation errors yourself, or if you have excluded fields from the

:class:`~django.forms.ModelForm` that require validation.

.. warning::

    Constraints containing :class:`~django.db.models.JSONField` may not raise

    validation errors as key, index, and path transforms have many

    database-specific caveats. This :ticket:`may be fully supported later

    <34059>`.

    You should always check that there are no log messages, in the

    ``django.db.models`` logger, like *"Got a database error calling check() on

    …"* to confirm it's validated properly.

.. versionchanged:: 4.1

    In older versions, constraints were not checked during the model

    validation.

.. method:: Model.full_clean(exclude=None, validate_unique=True, validate_constraints=True)

This method calls :meth:`Model.clean_fields()`, :meth:`Model.clean()`,

:meth:`Model.validate_unique()` (if ``validate_unique`` is ``True``),  and

:meth:`Model.validate_constraints()` (if ``validate_constraints`` is ``True``)

in that order and raises a :exc:`~django.core.exceptions.ValidationError` that

has a ``message_dict`` attribute containing errors from all four stages.

The optional ``exclude`` argument can be used to provide a ``set`` of field

names that can be excluded from validation and cleaning.

:class:`~django.forms.ModelForm` uses this argument to exclude fields that

aren't present on your form from being validated since any errors raised could

not be corrected by the user.

Note that ``full_clean()`` will *not* be called automatically when you call

your model's :meth:`~Model.save()` method. You'll need to call it manually

when you want to run one-step model validation for your own manually created

models. For example::

    from django.core.exceptions import ValidationError

    try:

        article.full_clean()

    except ValidationError as e:

        # Do something based on the errors contained in e.message_dict.

        # Display them to a user, or handle them programmatically.

        pass

The first step ``full_clean()`` performs is to clean each individual field.

.. versionchanged:: 4.1

    The ``validate_constraints`` argument was added.

.. versionchanged:: 4.1

    An ``exclude`` value is now converted to a ``set`` rather than a ``list``.

.. method:: Model.clean_fields(exclude=None)

This method will validate all fields on your model. The optional ``exclude``

argument lets you provide a ``set`` of field names to exclude from validation.

It will raise a :exc:`~django.core.exceptions.ValidationError` if any fields

fail validation.

The second step ``full_clean()`` performs is to call :meth:`Model.clean()`.

This method should be overridden to perform custom validation on your model.

.. method:: Model.clean()

This method should be used to provide custom model validation, and to modify

attributes on your model if desired. For instance, you could use it to

automatically provide a value for a field, or to do validation that requires

access to more than a single field::

    import datetime

    from django.core.exceptions import ValidationError

    from django.db import models

    from django.utils.translation import gettext_lazy as _

    class Article(models.Model):

        ...

        def clean(self):

            # Don't allow draft entries to have a pub_date.

            if self.status == 'draft' and self.pub_date is not None:

                raise ValidationError(_('Draft entries may not have a publication date.'))

            # Set the pub_date for published items if it hasn't been set already.

            if self.status == 'published' and self.pub_date is None:

                self.pub_date = datetime.date.today()

Note, however, that like :meth:`Model.full_clean()`, a model's ``clean()``

method is not invoked when you call your model's :meth:`~Model.save()` method.

In the above example, the :exc:`~django.core.exceptions.ValidationError`

exception raised by ``Model.clean()`` was instantiated with a string, so it

will be stored in a special error dictionary key,

:data:`~django.core.exceptions.NON_FIELD_ERRORS`. This key is used for errors

that are tied to the entire model instead of to a specific field::

    from django.core.exceptions import NON_FIELD_ERRORS, ValidationError

    try:

        article.full_clean()

    except ValidationError as e:

        non_field_errors = e.message_dict[NON_FIELD_ERRORS]

To assign exceptions to a specific field, instantiate the

:exc:`~django.core.exceptions.ValidationError` with a dictionary, where the

keys are the field names. We could update the previous example to assign the

error to the ``pub_date`` field::

    class Article(models.Model):

        ...

        def clean(self):

            # Don't allow draft entries to have a pub_date.

            if self.status == 'draft' and self.pub_date is not None:

                raise ValidationError({'pub_date': _('Draft entries may not have a publication date.')})

            ...

If you detect errors in multiple fields during ``Model.clean()``, you can also

pass a dictionary mapping field names to errors::

    raise ValidationError({

        'title': ValidationError(_('Missing title.'), code='required'),

        'pub_date': ValidationError(_('Invalid date.'), code='invalid'),

    })

Then, ``full_clean()`` will check unique constraints on your model.

.. admonition:: How to raise field-specific validation errors if those fields don't appear in a ``ModelForm``

    You can't raise validation errors in ``Model.clean()`` for fields that

    don't appear in a model form (a form may limit its fields using

    ``Meta.fields`` or ``Meta.exclude``). Doing so will raise a ``ValueError``

    because the validation error won't be able to be associated with the

    excluded field.

    To work around this dilemma, instead override :meth:`Model.clean_fields()

    <django.db.models.Model.clean_fields>` as it receives the list of fields

    that are excluded from validation. For example::

        class Article(models.Model):

            ...

            def clean_fields(self, exclude=None):

                super().clean_fields(exclude=exclude)

                if self.status == 'draft' and self.pub_date is not None:

                    if exclude and 'status' in exclude:

                        raise ValidationError(

                            _('Draft entries may not have a publication date.')

                        )

                    else:

                        raise ValidationError({

                            'status': _(

                                'Set status to draft if there is not a '

                                'publication date.'

                             ),

                        })

.. method:: Model.validate_unique(exclude=None)

This method is similar to :meth:`~Model.clean_fields`, but validates

uniqueness constraints defined via :attr:`.Field.unique`,

:attr:`.Field.unique_for_date`, :attr:`.Field.unique_for_month`,

:attr:`.Field.unique_for_year`, or :attr:`Meta.unique_together

<django.db.models.Options.unique_together>` on your model instead of individual

field values. The optional ``exclude`` argument allows you to provide a ``set``

of field names to exclude from validation. It will raise a

:exc:`~django.core.exceptions.ValidationError` if any fields fail validation.

:class:`~django.db.models.UniqueConstraint`\s defined in the

:attr:`Meta.constraints <django.db.models.Options.constraints>` are validated

by :meth:`Model.validate_constraints`.

Note that if you provide an ``exclude`` argument to ``validate_unique()``, any

:attr:`~django.db.models.Options.unique_together` constraint involving one of

the fields you provided will not be checked.

Finally, ``full_clean()`` will check any other constraints on your model.

.. versionchanged:: 4.1

    In older versions, :class:`~django.db.models.UniqueConstraint`\s were

    validated by ``validate_unique()``.

.. method:: Model.validate_constraints(exclude=None)

.. versionadded:: 4.1

This method validates all constraints defined in

:attr:`Meta.constraints <django.db.models.Options.constraints>`. The

optional ``exclude`` argument allows you to provide a ``set`` of field names to

exclude from validation. It will raise a

:exc:`~django.core.exceptions.ValidationError` if any constraints fail

validation.

Saving objects

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

To save an object back to the database, call ``save()``:

.. method:: Model.save(force_insert=False, force_update=False, using=DEFAULT_DB_ALIAS, update_fields=None)

For details on using the ``force_insert`` and ``force_update`` arguments, see

:ref:`ref-models-force-insert`. Details about the ``update_fields`` argument

can be found in the :ref:`ref-models-update-fields` section.

If you want customized saving behavior, you can override this ``save()``

method. See :ref:`overriding-model-methods` for more details.

The model save process also has some subtleties; see the sections below.

Auto-incrementing primary keys

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

If a model has an :class:`~django.db.models.AutoField` — an auto-incrementing

primary key — then that auto-incremented value will be calculated and saved as

an attribute on your object the first time you call ``save()``::

    >>> b2 = Blog(name='Cheddar Talk', tagline='Thoughts on cheese.')

    >>> b2.id     # Returns None, because b2 doesn't have an ID yet.

    >>> b2.save()

    >>> b2.id     # Returns the ID of your new object.

There's no way to tell what the value of an ID will be before you call

``save()``, because that value is calculated by your database, not by Django.

For convenience, each model has an :class:`~django.db.models.AutoField` named

``id`` by default unless you explicitly specify ``primary_key=True`` on a field

in your model. See the documentation for :class:`~django.db.models.AutoField`

for more details.

The ``pk`` property

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

.. attribute:: Model.pk

Regardless of whether you define a primary key field yourself, or let Django

supply one for you, each model will have a property called ``pk``. It behaves

like a normal attribute on the model, but is actually an alias for whichever

attribute is the primary key field for the model. You can read and set this

value, just as you would for any other attribute, and it will update the

correct field in the model.

Explicitly specifying auto-primary-key values

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

If a model has an :class:`~django.db.models.AutoField` but you want to define a

new object's ID explicitly when saving, define it explicitly before saving,

rather than relying on the auto-assignment of the ID::

    >>> b3 = Blog(id=3, name='Cheddar Talk', tagline='Thoughts on cheese.')

    >>> b3.id     # Returns 3.

    >>> b3.save()

    >>> b3.id     # Returns 3.

If you assign auto-primary-key values manually, make sure not to use an

already-existing primary-key value! If you create a new object with an explicit

primary-key value that already exists in the database, Django will assume you're

changing the existing record rather than creating a new one.

Given the above ``'Cheddar Talk'`` blog example, this example would override the

previous record in the database::

    b4 = Blog(id=3, name='Not Cheddar', tagline='Anything but cheese.')

    b4.save()  # Overrides the previous blog with ID=3!

See `How Django knows to UPDATE vs. INSERT`_, below, for the reason this

happens.

Explicitly specifying auto-primary-key values is mostly useful for bulk-saving

objects, when you're confident you won't have primary-key collision.

If you're using PostgreSQL, the sequence associated with the primary key might

need to be updated; see :ref:`manually-specified-autoincrement-pk`.

What happens when you save?

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

When you save an object, Django performs the following steps:

#. **Emit a pre-save signal.** The :data:`~django.db.models.signals.pre_save`

   signal is sent, allowing any functions listening for that signal to do

   something.

#. **Preprocess the data.** Each field's

   :meth:`~django.db.models.Field.pre_save` method is called to perform any

   automated data modification that's needed. For example, the date/time fields

   override ``pre_save()`` to implement

   :attr:`~django.db.models.DateField.auto_now_add` and

   :attr:`~django.db.models.DateField.auto_now`.

#. **Prepare the data for the database.** Each field's

   :meth:`~django.db.models.Field.get_db_prep_save` method is asked to provide

   its current value in a data type that can be written to the database.

   Most fields don't require data preparation. Simple data types, such as

   integers and strings, are 'ready to write' as a Python object. However, more

   complex data types often require some modification.

   For example, :class:`~django.db.models.DateField` fields use a Python

   ``datetime`` object to store data. Databases don't store ``datetime``

   objects, so the field value must be converted into an ISO-compliant date

   string for insertion into the database.

#. **Insert the data into the database.** The preprocessed, prepared data is

   composed into an SQL statement for insertion into the database.

#. **Emit a post-save signal.** The :data:`~django.db.models.signals.post_save`

   signal is sent, allowing any functions listening for that signal to do

   something.

How Django knows to UPDATE vs. INSERT

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

You may have noticed Django database objects use the same ``save()`` method

for creating and changing objects. Django abstracts the need to use ``INSERT``

or ``UPDATE`` SQL statements. Specifically, when you call ``save()`` and the

object's primary key attribute does **not** define a

:attr:`~django.db.models.Field.default`, Django follows this algorithm:

* If the object's primary key attribute is set to a value that evaluates to

  ``True`` (i.e., a value other than ``None`` or the empty string), Django

  executes an ``UPDATE``.

* If the object's primary key attribute is *not* set or if the ``UPDATE``

  didn't update anything (e.g. if primary key is set to a value that doesn't

  exist in the database), Django executes an ``INSERT``.

If the object's primary key attribute defines a

:attr:`~django.db.models.Field.default` then Django executes an ``UPDATE`` if

it is an existing model instance and primary key is set to a value that exists

in the database. Otherwise, Django executes an ``INSERT``.

The one gotcha here is that you should be careful not to specify a primary-key

value explicitly when saving new objects, if you cannot guarantee the

primary-key value is unused. For more on this nuance, see `Explicitly specifying

auto-primary-key values`_ above and `Forcing an INSERT or UPDATE`_ below.

In Django 1.5 and earlier, Django did a ``SELECT`` when the primary key

attribute was set. If the ``SELECT`` found a row, then Django did an ``UPDATE``,

otherwise it did an ``INSERT``. The old algorithm results in one more query in

the ``UPDATE`` case. There are some rare cases where the database doesn't

report that a row was updated even if the database contains a row for the

object's primary key value. An example is the PostgreSQL ``ON UPDATE`` trigger

which returns ``NULL``. In such cases it is possible to revert to the old

algorithm by setting the :attr:`~django.db.models.Options.select_on_save`

option to ``True``.

.. _ref-models-force-insert:

Forcing an INSERT or UPDATE

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

In some rare circumstances, it's necessary to be able to force the

:meth:`~Model.save()` method to perform an SQL ``INSERT`` and not fall back to

doing an ``UPDATE``. Or vice-versa: update, if possible, but not insert a new

row. In these cases you can pass the ``force_insert=True`` or

``force_update=True`` parameters to the :meth:`~Model.save()` method.

Passing both parameters is an error: you cannot both insert *and* update at the

same time!

It should be very rare that you'll need to use these parameters. Django will

almost always do the right thing and trying to override that will lead to

errors that are difficult to track down. This feature is for advanced use

only.

Using ``update_fields`` will force an update similarly to ``force_update``.

.. _ref-models-field-updates-using-f-expressions:

Updating attributes based on existing fields

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

Sometimes you'll need to perform a simple arithmetic task on a field, such

as incrementing or decrementing the current value. One way of achieving this is

doing the arithmetic in Python like::

    >>> product = Product.objects.get(name='Venezuelan Beaver Cheese')

    >>> product.number_sold += 1

    >>> product.save()

If the old ``number_sold`` value retrieved from the database was 10, then

the value of 11 will be written back to the database.

The process can be made robust, :ref:`avoiding a race condition

<avoiding-race-conditions-using-f>`, as well as slightly faster by expressing

the update relative to the original field value, rather than as an explicit

assignment of a new value. Django provides :class:`F expressions

<django.db.models.F>` for performing this kind of relative update. Using

:class:`F expressions <django.db.models.F>`, the previous example is expressed

as::

    >>> from django.db.models import F

    >>> product = Product.objects.get(name='Venezuelan Beaver Cheese')

    >>> product.number_sold = F('number_sold') + 1

    >>> product.save()

For more details, see the documentation on :class:`F expressions

<django.db.models.F>` and their :ref:`use in update queries

<topics-db-queries-update>`.

.. _ref-models-update-fields:

Specifying which fields to save

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

If ``save()`` is passed a list of field names in keyword argument

``update_fields``, only the fields named in that list will be updated.

This may be desirable if you want to update just one or a few fields on

an object. There will be a slight performance benefit from preventing

all of the model fields from being updated in the database. For example::

    product.name = 'Name changed again'

    product.save(update_fields=['name'])

The ``update_fields`` argument can be any iterable containing strings. An

empty ``update_fields`` iterable will skip the save. A value of ``None`` will

perform an update on all fields.

Specifying ``update_fields`` will force an update.

When saving a model fetched through deferred model loading

(:meth:`~django.db.models.query.QuerySet.only()` or

:meth:`~django.db.models.query.QuerySet.defer()`) only the fields loaded

from the DB will get updated. In effect there is an automatic

``update_fields`` in this case. If you assign or change any deferred field

value, the field will be added to the updated fields.

.. admonition:: ``Field.pre_save()`` and ``update_fields``

    If ``update_fields`` is passed in, only the

    :meth:`~django.db.models.Field.pre_save` methods of the ``update_fields``

    are called. For example, this means that date/time fields with

    ``auto_now=True`` will not be updated unless they are included in the

    ``update_fields``.

Deleting objects

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

.. method:: Model.delete(using=DEFAULT_DB_ALIAS, keep_parents=False)

Issues an SQL ``DELETE`` for the object. This only deletes the object in the

database; the Python instance will still exist and will still have data in

its fields, except for the primary key set to ``None``. This method returns the

number of objects deleted and a dictionary with the number of deletions per

object type.

For more details, including how to delete objects in bulk, see

:ref:`topics-db-queries-delete`.

If you want customized deletion behavior, you can override the ``delete()``

method. See :ref:`overriding-model-methods` for more details.

Sometimes with :ref:`multi-table inheritance <multi-table-inheritance>` you may

want to delete only a child model's data. Specifying ``keep_parents=True`` will

keep the parent model's data.

Pickling objects

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

When you :mod:`pickle` a model, its current state is pickled. When you unpickle

it, it'll contain the model instance at the moment it was pickled, rather than

the data that's currently in the database.

.. admonition:: You can't share pickles between versions

    Pickles of models are only valid for the version of Django that

    was used to generate them. If you generate a pickle using Django

    version N, there is no guarantee that pickle will be readable with

    Django version N+1. Pickles should not be used as part of a long-term

    archival strategy.

    Since pickle compatibility errors can be difficult to diagnose, such as

    silently corrupted objects, a ``RuntimeWarning`` is raised when you try to

    unpickle a model in a Django version that is different than the one in

    which it was pickled.

.. _model-instance-methods:

Other model instance methods

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

A few object methods have special purposes.

``__str__()``

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

.. method:: Model.__str__()

The ``__str__()`` method is called whenever you call ``str()`` on an object.

Django uses ``str(obj)`` in a number of places. Most notably, to display an

object in the Django admin site and as the value inserted into a template when

it displays an object. Thus, you should always return a nice, human-readable

representation of the model from the ``__str__()`` method.

For example::

    from django.db import models

    class Person(models.Model):

        first_name = models.CharField(max_length=50)

        last_name = models.CharField(max_length=50)

        def __str__(self):

            return '%s %s' % (self.first_name, self.last_name)

``__eq__()``

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

.. method:: Model.__eq__()

The equality method is defined such that instances with the same primary

key value and the same concrete class are considered equal, except that

instances with a primary key value of ``None`` aren't equal to anything except

themselves. For proxy models, concrete class is defined as the model's first

non-proxy parent; for all other models it's simply the model's class.

For example::

    from django.db import models

    class MyModel(models.Model):

        id = models.AutoField(primary_key=True)

    class MyProxyModel(MyModel):

        class Meta:

            proxy = True

    class MultitableInherited(MyModel):

        pass

    # Primary keys compared

    MyModel(id=1) == MyModel(id=1)

    MyModel(id=1) != MyModel(id=2)

    # Primary keys are None

    MyModel(id=None) != MyModel(id=None)

    # Same instance

    instance = MyModel(id=None)

    instance == instance

    # Proxy model

    MyModel(id=1) == MyProxyModel(id=1)

    # Multi-table inheritance

    MyModel(id=1) != MultitableInherited(id=1)

``__hash__()``

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

.. method:: Model.__hash__()

The ``__hash__()`` method is based on the instance's primary key value. It

is effectively ``hash(obj.pk)``. If the instance doesn't have a primary key

value then a ``TypeError`` will be raised (otherwise the ``__hash__()``

method would return different values before and after the instance is

saved, but changing the :meth:`~object.__hash__` value of an instance is

forbidden in Python.

``get_absolute_url()``

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

.. method:: Model.get_absolute_url()

Define a ``get_absolute_url()`` method to tell Django how to calculate the

canonical URL for an object. To callers, this method should appear to return a

string that can be used to refer to the object over HTTP.

For example::

    def get_absolute_url(self):

        return "/people/%i/" % self.id

While this code is correct and simple, it may not be the most portable way to

to write this kind of method. The :func:`~django.urls.reverse` function is

usually the best approach.

For example::

    def get_absolute_url(self):

        from django.urls import reverse

        return reverse('people-detail', kwargs={'pk' : self.pk})

One place Django uses ``get_absolute_url()`` is in the admin app. If an object

defines this method, the object-editing page will have a "View on site" link

that will jump you directly to the object's public view, as given by

``get_absolute_url()``.

Similarly, a couple of other bits of Django, such as the :doc:`syndication feed

framework </ref/contrib/syndication>`, use ``get_absolute_url()`` when it is

defined. If it makes sense for your model's instances to each have a unique

URL, you should define ``get_absolute_url()``.

.. warning::

    You should avoid building the URL from unvalidated user input, in order to

    reduce possibilities of link or redirect poisoning::

        def get_absolute_url(self):

            return '/%s/' % self.name

    If ``self.name`` is ``'/example.com'`` this returns ``'//example.com/'``

    which, in turn, is a valid schema relative URL but not the expected

    ``'/%2Fexample.com/'``.

It's good practice to use ``get_absolute_url()`` in templates, instead of

hard-coding your objects' URLs. For example, this template code is bad:

.. code-block:: html+django

    <!-- BAD template code. Avoid! -->

    <a href="/people/{{ object.id }}/">{{ object.name }}</a>

This template code is much better:

.. code-block:: html+django

    <a href="{{ object.get_absolute_url }}">{{ object.name }}</a>

The logic here is that if you change the URL structure of your objects, even

for something small like correcting a spelling error, you don't want to have to

track down every place that the URL might be created. Specify it once, in

``get_absolute_url()`` and have all your other code call that one place.

.. note::

    The string you return from ``get_absolute_url()`` **must** contain only

    ASCII characters (required by the URI specification, :rfc:`2396#section-2`)

    and be URL-encoded, if necessary.

    Code and templates calling ``get_absolute_url()`` should be able to use the

    result directly without any further processing. You may wish to use the

    ``django.utils.encoding.iri_to_uri()`` function to help with this if you

    are using strings containing characters outside the ASCII range.

Extra instance methods

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

In addition to :meth:`~Model.save()`, :meth:`~Model.delete()`, a model object

might have some of the following methods:

.. method:: Model.get_FOO_display()

For every field that has :attr:`~django.db.models.Field.choices` set, the

object will have a ``get_FOO_display()`` method, where ``FOO`` is the name of

the field. This method returns the "human-readable" value of the field.

For example::

    from django.db import models

    class Person(models.Model):

        SHIRT_SIZES = (

            ('S', 'Small'),

            ('M', 'Medium'),

            ('L', 'Large'),

        )

        name = models.CharField(max_length=60)

        shirt_size = models.CharField(max_length=2, choices=SHIRT_SIZES)

::

    >>> p = Person(name="Fred Flintstone", shirt_size="L")

    >>> p.save()

    >>> p.shirt_size

    'L'

    >>> p.get_shirt_size_display()

    'Large'

.. method:: Model.get_next_by_FOO(**kwargs)

.. method:: Model.get_previous_by_FOO(**kwargs)

For every :class:`~django.db.models.DateField` and

:class:`~django.db.models.DateTimeField` that does not have :attr:`null=True

<django.db.models.Field.null>`, the object will have ``get_next_by_FOO()`` and

``get_previous_by_FOO()`` methods, where ``FOO`` is the name of the field. This

returns the next and previous object with respect to the date field, raising

a :exc:`~django.db.models.Model.DoesNotExist` exception when appropriate.

Both of these methods will perform their queries using the default

manager for the model. If you need to emulate filtering used by a

custom manager, or want to perform one-off custom filtering, both

methods also accept optional keyword arguments, which should be in the

format described in :ref:`Field lookups <field-lookups>`.

Note that in the case of identical date values, these methods will use the

primary key as a tie-breaker. This guarantees that no records are skipped or

duplicated. That also means you cannot use those methods on unsaved objects.

.. admonition:: Overriding extra instance methods

    In most cases overriding or inheriting ``get_FOO_display()``,

    ``get_next_by_FOO()``, and ``get_previous_by_FOO()`` should work as

    expected. Since they are added by the metaclass however, it is not

    practical to account for all possible inheritance structures. In more

    complex cases you should override ``Field.contribute_to_class()`` to set

    the methods you need.

Other attributes

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

``_state``

----------

.. attribute:: Model._state

    The ``_state`` attribute refers to a ``ModelState`` object that tracks

    the lifecycle of the model instance.

    The ``ModelState`` object has two attributes: ``adding``, a flag which is

    ``True`` if the model has not been saved to the database yet, and ``db``,

    a string referring to the database alias the instance was loaded from or

    saved to.

    Newly instantiated instances have ``adding=True`` and ``db=None``,

    since they are yet to be saved. Instances fetched from a ``QuerySet``

    will have ``adding=False`` and ``db`` set to the alias of the associated

    database.