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

Writing your first Django app, part 2

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

This tutorial begins where :doc:`Tutorial 1 </intro/tutorial01>` left off.

We'll set up the database, create your first model, and get a quick

introduction to Django's automatically-generated admin site.

.. admonition:: Where to get help:

    If you're having trouble going through this tutorial, please head over to

    the :doc:`Getting Help</faq/help>` section of the FAQ.

Database setup

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

Now, open up :file:`mysite/settings.py`. It's a normal Python module with

module-level variables representing Django settings.

By default, the configuration uses SQLite. If you're new to databases, or

you're just interested in trying Django, this is the easiest choice. SQLite is

included in Python, so you won't need to install anything else to support your

database. When starting your first real project, however, you may want to use a

more scalable database like PostgreSQL, to avoid database-switching headaches

down the road.

If you wish to use another database, install the appropriate :ref:`database

bindings <database-installation>` and change the following keys in the

:setting:`DATABASES` ``'default'`` item to match your database connection

settings:

* :setting:`ENGINE <DATABASE-ENGINE>` -- Either

  ``'django.db.backends.sqlite3'``,

  ``'django.db.backends.postgresql'``,

  ``'django.db.backends.mysql'``, or

  ``'django.db.backends.oracle'``. Other backends are :ref:`also available

  <third-party-notes>`.

* :setting:`NAME` -- The name of your database. If you're using SQLite, the

  database will be a file on your computer; in that case, :setting:`NAME`

  should be the full absolute path, including filename, of that file. The

  default value, ``BASE_DIR / 'db.sqlite3'``, will store the file in your

  project directory.

If you are not using SQLite as your database, additional settings such as

:setting:`USER`, :setting:`PASSWORD`, and :setting:`HOST` must be added.

For more details, see the reference documentation for :setting:`DATABASES`.

.. admonition:: For databases other than SQLite

    If you're using a database besides SQLite, make sure you've created a

    database by this point. Do that with "``CREATE DATABASE database_name;``"

    within your database's interactive prompt.

    Also make sure that the database user provided in :file:`mysite/settings.py`

    has "create database" privileges. This allows automatic creation of a

    :ref:`test database <the-test-database>` which will be needed in a later

    tutorial.

    If you're using SQLite, you don't need to create anything beforehand - the

    database file will be created automatically when it is needed.

While you're editing :file:`mysite/settings.py`, set :setting:`TIME_ZONE` to

your time zone.

Also, note the :setting:`INSTALLED_APPS` setting at the top of the file. That

holds the names of all Django applications that are activated in this Django

instance. Apps can be used in multiple projects, and you can package and

distribute them for use by others in their projects.

By default, :setting:`INSTALLED_APPS` contains the following apps, all of which

come with Django:

* :mod:`django.contrib.admin` -- The admin site. You'll use it shortly.

* :mod:`django.contrib.auth` -- An authentication system.

* :mod:`django.contrib.contenttypes` -- A framework for content types.

* :mod:`django.contrib.sessions` -- A session framework.

* :mod:`django.contrib.messages` -- A messaging framework.

* :mod:`django.contrib.staticfiles` -- A framework for managing

  static files.

These applications are included by default as a convenience for the common case.

Some of these applications make use of at least one database table, though,

so we need to create the tables in the database before we can use them. To do

that, run the following command:

.. console::

    $ python manage.py migrate

The :djadmin:`migrate` command looks at the :setting:`INSTALLED_APPS` setting

and creates any necessary database tables according to the database settings

in your :file:`mysite/settings.py` file and the database migrations shipped

with the app (we'll cover those later). You'll see a message for each

migration it applies. If you're interested, run the command-line client for your

database and type ``\dt`` (PostgreSQL), ``SHOW TABLES;`` (MariaDB, MySQL),

``.tables`` (SQLite), or ``SELECT TABLE_NAME FROM USER_TABLES;`` (Oracle) to

display the tables Django created.

.. admonition:: For the minimalists

    Like we said above, the default applications are included for the common

    case, but not everybody needs them. If you don't need any or all of them,

    feel free to comment-out or delete the appropriate line(s) from

    :setting:`INSTALLED_APPS` before running :djadmin:`migrate`. The

    :djadmin:`migrate` command will only run migrations for apps in

    :setting:`INSTALLED_APPS`.

.. _creating-models:

Creating models

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

Now we'll define your models -- essentially, your database layout, with

additional metadata.

.. admonition:: Philosophy

   A model is the single, definitive source of information about your data. It

   contains the essential fields and behaviors of the data you're storing.

   Django follows the :ref:`DRY Principle <dry>`. The goal is to define your

   data model in one place and automatically derive things from it.

   This includes the migrations - unlike in Ruby On Rails, for example, migrations

   are entirely derived from your models file, and are essentially a

   history that Django can roll through to update your database schema to

   match your current models.

In our poll app, we'll create two models: ``Question`` and ``Choice``. A

``Question`` has a question and a publication date. A ``Choice`` has two

fields: the text of the choice and a vote tally. Each ``Choice`` is associated

with a ``Question``.

These concepts are represented by Python classes. Edit the

:file:`polls/models.py` file so it looks like this:

.. code-block:: python

    :caption: ``polls/models.py``

    from django.db import models

    class Question(models.Model):

        question_text = models.CharField(max_length=200)

        pub_date = models.DateTimeField('date published')

    class Choice(models.Model):

        question = models.ForeignKey(Question, on_delete=models.CASCADE)

        choice_text = models.CharField(max_length=200)

        votes = models.IntegerField(default=0)

Here, each model is represented by a class that subclasses

:class:`django.db.models.Model`. Each model has a number of class variables,

each of which represents a database field in the model.

Each field is represented by an instance of a :class:`~django.db.models.Field`

class -- e.g., :class:`~django.db.models.CharField` for character fields and

:class:`~django.db.models.DateTimeField` for datetimes. This tells Django what

type of data each field holds.

The name of each :class:`~django.db.models.Field` instance (e.g.

``question_text`` or ``pub_date``) is the field's name, in machine-friendly

format. You'll use this value in your Python code, and your database will use

it as the column name.

You can use an optional first positional argument to a

:class:`~django.db.models.Field` to designate a human-readable name. That's used

in a couple of introspective parts of Django, and it doubles as documentation.

If this field isn't provided, Django will use the machine-readable name. In this

example, we've only defined a human-readable name for ``Question.pub_date``.

For all other fields in this model, the field's machine-readable name will

suffice as its human-readable name.

Some :class:`~django.db.models.Field` classes have required arguments.

:class:`~django.db.models.CharField`, for example, requires that you give it a

:attr:`~django.db.models.CharField.max_length`. That's used not only in the

database schema, but in validation, as we'll soon see.

A :class:`~django.db.models.Field` can also have various optional arguments; in

this case, we've set the :attr:`~django.db.models.Field.default` value of

``votes`` to 0.

Finally, note a relationship is defined, using

:class:`~django.db.models.ForeignKey`. That tells Django each ``Choice`` is

related to a single ``Question``. Django supports all the common database

relationships: many-to-one, many-to-many, and one-to-one.

Activating models

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

That small bit of model code gives Django a lot of information. With it, Django

is able to:

* Create a database schema (``CREATE TABLE`` statements) for this app.

* Create a Python database-access API for accessing ``Question`` and ``Choice`` objects.

But first we need to tell our project that the ``polls`` app is installed.

.. admonition:: Philosophy

    Django apps are "pluggable": You can use an app in multiple projects, and

    you can distribute apps, because they don't have to be tied to a given

    Django installation.

To include the app in our project, we need to add a reference to its

configuration class in the :setting:`INSTALLED_APPS` setting. The

``PollsConfig`` class is in the :file:`polls/apps.py` file, so its dotted path

is ``'polls.apps.PollsConfig'``. Edit the :file:`mysite/settings.py` file and

add that dotted path to the :setting:`INSTALLED_APPS` setting. It'll look like

this:

.. code-block:: python

    :caption: ``mysite/settings.py``

    INSTALLED_APPS = [

        'polls.apps.PollsConfig',

        'django.contrib.admin',

        'django.contrib.auth',

        'django.contrib.contenttypes',

        'django.contrib.sessions',

        'django.contrib.messages',

        'django.contrib.staticfiles',

    ]

Now Django knows to include the ``polls`` app. Let's run another command:

.. console::

    $ python manage.py makemigrations polls

You should see something similar to the following:

.. code-block:: text

    Migrations for 'polls':

      polls/migrations/0001_initial.py

        - Create model Question

        - Create model Choice

By running ``makemigrations``, you're telling Django that you've made

some changes to your models (in this case, you've made new ones) and that

you'd like the changes to be stored as a *migration*.

Migrations are how Django stores changes to your models (and thus your

database schema) - they're files on disk. You can read the migration for your

new model if you like; it's the file ``polls/migrations/0001_initial.py``.

Don't worry, you're not expected to read them every time Django makes one, but

they're designed to be human-editable in case you want to manually tweak how

Django changes things.

There's a command that will run the migrations for you and manage your database

schema automatically - that's called :djadmin:`migrate`, and we'll come to it in a

moment - but first, let's see what SQL that migration would run. The

:djadmin:`sqlmigrate` command takes migration names and returns their SQL:

.. console::

    $ python manage.py sqlmigrate polls 0001

You should see something similar to the following (we've reformatted it for

readability):

.. code-block:: sql

    BEGIN;

    --

    -- Create model Question

    --

    CREATE TABLE "polls_question" (

        "id" bigint NOT NULL PRIMARY KEY GENERATED BY DEFAULT AS IDENTITY,

        "question_text" varchar(200) NOT NULL,

        "pub_date" timestamp with time zone NOT NULL

    );

    --

    -- Create model Choice

    --

    CREATE TABLE "polls_choice" (

        "id" bigint NOT NULL PRIMARY KEY GENERATED BY DEFAULT AS IDENTITY,

        "choice_text" varchar(200) NOT NULL,

        "votes" integer NOT NULL,

        "question_id" bigint NOT NULL

    );

    ALTER TABLE "polls_choice"

      ADD CONSTRAINT "polls_choice_question_id_c5b4b260_fk_polls_question_id"

        FOREIGN KEY ("question_id")

        REFERENCES "polls_question" ("id")

        DEFERRABLE INITIALLY DEFERRED;

    CREATE INDEX "polls_choice_question_id_c5b4b260" ON "polls_choice" ("question_id");

    COMMIT;

Note the following:

* The exact output will vary depending on the database you are using. The

  example above is generated for PostgreSQL.

* Table names are automatically generated by combining the name of the app

  (``polls``) and the lowercase name of the model -- ``question`` and

  ``choice``. (You can override this behavior.)

* Primary keys (IDs) are added automatically. (You can override this, too.)

* By convention, Django appends ``"_id"`` to the foreign key field name.

  (Yes, you can override this, as well.)

* The foreign key relationship is made explicit by a ``FOREIGN KEY``

  constraint. Don't worry about the ``DEFERRABLE`` parts; it's telling

  PostgreSQL to not enforce the foreign key until the end of the transaction.

* It's tailored to the database you're using, so database-specific field types

  such as ``auto_increment`` (MySQL), ``bigint PRIMARY KEY GENERATED BY DEFAULT

  AS IDENTITY`` (PostgreSQL), or ``integer primary key autoincrement`` (SQLite)

  are handled for you automatically. Same goes for the quoting of field names

  -- e.g., using double quotes or single quotes.

* The :djadmin:`sqlmigrate` command doesn't actually run the migration on your

  database - instead, it prints it to the screen so that you can see what SQL

  Django thinks is required. It's useful for checking what Django is going to

  do or if you have database administrators who require SQL scripts for

  changes.

If you're interested, you can also run

:djadmin:`python manage.py check <check>`; this checks for any problems in

your project without making migrations or touching the database.

Now, run :djadmin:`migrate` again to create those model tables in your database:

.. console::

    $ python manage.py migrate

    Operations to perform:

      Apply all migrations: admin, auth, contenttypes, polls, sessions

    Running migrations:

      Rendering model states... DONE

      Applying polls.0001_initial... OK

The :djadmin:`migrate` command takes all the migrations that haven't been

applied (Django tracks which ones are applied using a special table in your

database called ``django_migrations``) and runs them against your database -

essentially, synchronizing the changes you made to your models with the schema

in the database.

Migrations are very powerful and let you change your models over time, as you

develop your project, without the need to delete your database or tables and

make new ones - it specializes in upgrading your database live, without

losing data. We'll cover them in more depth in a later part of the tutorial,

but for now, remember the three-step guide to making model changes:

* Change your models (in ``models.py``).

* Run :djadmin:`python manage.py makemigrations <makemigrations>` to create

  migrations for those changes

* Run :djadmin:`python manage.py migrate <migrate>` to apply those changes to

  the database.

The reason that there are separate commands to make and apply migrations is

because you'll commit migrations to your version control system and ship them

with your app; they not only make your development easier, they're also

usable by other developers and in production.

Read the :doc:`django-admin documentation </ref/django-admin>` for full

information on what the ``manage.py`` utility can do.

Playing with the API

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

Now, let's hop into the interactive Python shell and play around with the free

API Django gives you. To invoke the Python shell, use this command:

.. console::

    $ python manage.py shell

We're using this instead of simply typing "python", because :file:`manage.py`

sets the :envvar:`DJANGO_SETTINGS_MODULE` environment variable, which gives

Django the Python import path to your :file:`mysite/settings.py` file.

Once you're in the shell, explore the :doc:`database API </topics/db/queries>`::

    >>> from polls.models import Choice, Question  # Import the model classes we just wrote.

    # No questions are in the system yet.

    >>> Question.objects.all()

    <QuerySet []>

    # Create a new Question.

    # Support for time zones is enabled in the default settings file, so

    # Django expects a datetime with tzinfo for pub_date. Use timezone.now()

    # instead of datetime.datetime.now() and it will do the right thing.

    >>> from django.utils import timezone

    >>> q = Question(question_text="What's new?", pub_date=timezone.now())

    # Save the object into the database. You have to call save() explicitly.

    >>> q.save()

    # Now it has an ID.

    >>> q.id

    1

    # Access model field values via Python attributes.

    >>> q.question_text

    "What's new?"

    >>> q.pub_date

    datetime.datetime(2012, 2, 26, 13, 0, 0, 775217, tzinfo=datetime.timezone.utc)

    # Change values by changing the attributes, then calling save().

    >>> q.question_text = "What's up?"

    >>> q.save()

    # objects.all() displays all the questions in the database.

    >>> Question.objects.all()

    <QuerySet [<Question: Question object (1)>]>

Wait a minute. ``<Question: Question object (1)>`` isn't a helpful

representation of this object. Let's fix that by editing the ``Question`` model

(in the ``polls/models.py`` file) and adding a

:meth:`~django.db.models.Model.__str__` method to both ``Question`` and

``Choice``:

.. code-block:: python

    :caption: ``polls/models.py``

    from django.db import models

    class Question(models.Model):

        # ...

        def __str__(self):

            return self.question_text

    class Choice(models.Model):

        # ...

        def __str__(self):

            return self.choice_text

It's important to add :meth:`~django.db.models.Model.__str__` methods to your

models, not only for your own convenience when dealing with the interactive

prompt, but also because objects' representations are used throughout Django's

automatically-generated admin.

.. _tutorial02-import-timezone:

Let's also add a custom method to this model:

.. code-block:: python

    :caption: ``polls/models.py``

    import datetime

    from django.db import models

    from django.utils import timezone

    class Question(models.Model):

        # ...

        def was_published_recently(self):

            return self.pub_date >= timezone.now() - datetime.timedelta(days=1)

Note the addition of ``import datetime`` and ``from django.utils import

timezone``, to reference Python's standard :mod:`datetime` module and Django's

time-zone-related utilities in :mod:`django.utils.timezone`, respectively. If

you aren't familiar with time zone handling in Python, you can learn more in

the :doc:`time zone support docs </topics/i18n/timezones>`.

Save these changes and start a new Python interactive shell by running

``python manage.py shell`` again::

    >>> from polls.models import Choice, Question

    # Make sure our __str__() addition worked.

    >>> Question.objects.all()

    <QuerySet [<Question: What's up?>]>

    # Django provides a rich database lookup API that's entirely driven by

    # keyword arguments.

    >>> Question.objects.filter(id=1)

    <QuerySet [<Question: What's up?>]>

    >>> Question.objects.filter(question_text__startswith='What')

    <QuerySet [<Question: What's up?>]>

    # Get the question that was published this year.

    >>> from django.utils import timezone

    >>> current_year = timezone.now().year

    >>> Question.objects.get(pub_date__year=current_year)

    <Question: What's up?>

    # Request an ID that doesn't exist, this will raise an exception.

    >>> Question.objects.get(id=2)

    Traceback (most recent call last):

        ...

    DoesNotExist: Question matching query does not exist.

    # Lookup by a primary key is the most common case, so Django provides a

    # shortcut for primary-key exact lookups.

    # The following is identical to Question.objects.get(id=1).

    >>> Question.objects.get(pk=1)

    <Question: What's up?>

    # Make sure our custom method worked.

    >>> q = Question.objects.get(pk=1)

    >>> q.was_published_recently()

    True

    # Give the Question a couple of Choices. The create call constructs a new

    # Choice object, does the INSERT statement, adds the choice to the set

    # of available choices and returns the new Choice object. Django creates

    # a set to hold the "other side" of a ForeignKey relation

    # (e.g. a question's choice) which can be accessed via the API.

    >>> q = Question.objects.get(pk=1)

    # Display any choices from the related object set -- none so far.

    >>> q.choice_set.all()

    <QuerySet []>

    # Create three choices.

    >>> q.choice_set.create(choice_text='Not much', votes=0)

    <Choice: Not much>

    >>> q.choice_set.create(choice_text='The sky', votes=0)

    <Choice: The sky>

    >>> c = q.choice_set.create(choice_text='Just hacking again', votes=0)

    # Choice objects have API access to their related Question objects.

    >>> c.question

    <Question: What's up?>

    # And vice versa: Question objects get access to Choice objects.

    >>> q.choice_set.all()

    <QuerySet [<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]>

    >>> q.choice_set.count()

    3

    # The API automatically follows relationships as far as you need.

    # Use double underscores to separate relationships.

    # This works as many levels deep as you want; there's no limit.

    # Find all Choices for any question whose pub_date is in this year

    # (reusing the 'current_year' variable we created above).

    >>> Choice.objects.filter(question__pub_date__year=current_year)

    <QuerySet [<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]>

    # Let's delete one of the choices. Use delete() for that.

    >>> c = q.choice_set.filter(choice_text__startswith='Just hacking')

    >>> c.delete()

For more information on model relations, see :doc:`Accessing related objects

</ref/models/relations>`. For more on how to use double underscores to perform

field lookups via the API, see :ref:`Field lookups <field-lookups-intro>`. For

full details on the database API, see our :doc:`Database API reference

</topics/db/queries>`.

Introducing the Django Admin

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

.. admonition:: Philosophy

    Generating admin sites for your staff or clients to add, change, and delete

    content is tedious work that doesn't require much creativity. For that

    reason, Django entirely automates creation of admin interfaces for models.

    Django was written in a newsroom environment, with a very clear separation

    between "content publishers" and the "public" site. Site managers use the

    system to add news stories, events, sports scores, etc., and that content is

    displayed on the public site. Django solves the problem of creating a

    unified interface for site administrators to edit content.

    The admin isn't intended to be used by site visitors. It's for site

    managers.

Creating an admin user

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

First we'll need to create a user who can login to the admin site. Run the

following command:

.. console::

    $ python manage.py createsuperuser

Enter your desired username and press enter.

.. code-block:: text

    Username: admin

You will then be prompted for your desired email address:

.. code-block:: text

    Email address: [email protected]

The final step is to enter your password. You will be asked to enter your

password twice, the second time as a confirmation of the first.

.. code-block:: text

    Password: **********

    Password (again): *********

    Superuser created successfully.

Start the development server

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

The Django admin site is activated by default. Let's start the development

server and explore it.

If the server is not running start it like so:

.. console::

    $ python manage.py runserver

Now, open a web browser and go to "/admin/" on your local domain -- e.g.,

http://127.0.0.1:8000/admin/. You should see the admin's login screen:

.. image:: _images/admin01.png

   :alt: Django admin login screen

Since :doc:`translation </topics/i18n/translation>` is turned on by default, if

you set :setting:`LANGUAGE_CODE`, the login screen will be displayed in the

given language (if Django has appropriate translations).

Enter the admin site

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

Now, try logging in with the superuser account you created in the previous step.

You should see the Django admin index page:

.. image:: _images/admin02.png

   :alt: Django admin index page

You should see a few types of editable content: groups and users. They are

provided by :mod:`django.contrib.auth`, the authentication framework shipped

by Django.

Make the poll app modifiable in the admin

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

But where's our poll app? It's not displayed on the admin index page.

Only one more thing to do: we need to tell the admin that ``Question`` objects

have an admin interface. To do this, open the :file:`polls/admin.py` file, and

edit it to look like this:

.. code-block:: python

    :caption: ``polls/admin.py``

    from django.contrib import admin

    from .models import Question

    admin.site.register(Question)

Explore the free admin functionality

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

Now that we've registered ``Question``, Django knows that it should be displayed on

the admin index page:

.. image:: _images/admin03t.png

   :alt: Django admin index page, now with polls displayed

Click "Questions". Now you're at the "change list" page for questions. This page

displays all the questions in the database and lets you choose one to change it.

There's the "What's up?" question we created earlier:

.. image:: _images/admin04t.png

   :alt: Polls change list page

Click the "What's up?" question to edit it:

.. image:: _images/admin05t.png

   :alt: Editing form for question object

Things to note here:

* The form is automatically generated from the ``Question`` model.

* The different model field types (:class:`~django.db.models.DateTimeField`,

  :class:`~django.db.models.CharField`) correspond to the appropriate HTML

  input widget. Each type of field knows how to display itself in the Django

  admin.

* Each :class:`~django.db.models.DateTimeField` gets free JavaScript

  shortcuts. Dates get a "Today" shortcut and calendar popup, and times get

  a "Now" shortcut and a convenient popup that lists commonly entered times.

The bottom part of the page gives you a couple of options:

* Save -- Saves changes and returns to the change-list page for this type of

  object.

* Save and continue editing -- Saves changes and reloads the admin page for

  this object.

* Save and add another -- Saves changes and loads a new, blank form for this

  type of object.

* Delete -- Displays a delete confirmation page.

If the value of "Date published" doesn't match the time when you created the

question in :doc:`Tutorial 1</intro/tutorial01>`, it probably

means you forgot to set the correct value for the :setting:`TIME_ZONE` setting.

Change it, reload the page and check that the correct value appears.

Change the "Date published" by clicking the "Today" and "Now" shortcuts. Then

click "Save and continue editing." Then click "History" in the upper right.

You'll see a page listing all changes made to this object via the Django admin,

with the timestamp and username of the person who made the change:

.. image:: _images/admin06t.png

   :alt: History page for question object

When you're comfortable with the models API and have familiarized yourself with

the admin site, read :doc:`part 3 of this tutorial</intro/tutorial03>` to learn

about how to add more views to our polls app.