django/docs/intro/tutorial01.txt

=====================================
Writing your first Django app, part 1
=====================================

Let's learn by example.

Throughout this tutorial, we'll walk you through the creation of a basic
poll application.

It'll consist of two parts:

* A public site that lets people view polls and vote in them.
* An admin site that lets you add, change and delete polls.

We'll assume you have :doc:`Django installed </intro/install>` already. You can
tell Django is installed and which version by running the following command:

.. code-block:: bash

    $ python -c "import django; print(django.get_version())"

If Django is installed, you should see the version of your installation. If it
isn't, you'll get an error telling "No module named django".

This tutorial is written for Django |version| and Python 3.3 or later. If the
Django version doesn't match, you can refer to the tutorial for your version
of Django by using the version switcher at the bottom right corner of this
page, or update Django to the newest version. If you are still using Python
2.7, you will need to adjust the code samples slightly, as described in
comments.

See :doc:`How to install Django </topics/install>` for advice on how to remove
older versions of Django and install a newer one.

.. admonition:: Where to get help:

    If you're having trouble going through this tutorial, please post a message
    to |django-users| or drop by `#django on irc.freenode.net
    <irc://irc.freenode.net/django>`_ to chat with other Django users who might
    be able to help.

Creating a project
==================

If this is your first time using Django, you'll have to take care of some
initial setup. Namely, you'll need to auto-generate some code that establishes a
Django :term:`project` -- a collection of settings for an instance of Django,
including database configuration, Django-specific options and
application-specific settings.

From the command line, ``cd`` into a directory where you'd like to store your
code, then run the following command:

.. code-block:: bash

   $ django-admin startproject mysite

This will create a ``mysite`` directory in your current directory. If it didn't
work, see :ref:`troubleshooting-django-admin`.

.. note::

    You'll need to avoid naming projects after built-in Python or Django
    components. In particular, this means you should avoid using names like
    ``django`` (which will conflict with Django itself) or ``test`` (which
    conflicts with a built-in Python package).

.. admonition:: Where should this code live?

    If your background is in plain old PHP (with no use of modern frameworks),
    you're probably used to putting code under the Web server's document root
    (in a place such as ``/var/www``). With Django, you don't do that. It's
    not a good idea to put any of this Python code within your Web server's
    document root, because it risks the possibility that people may be able
    to view your code over the Web. That's not good for security.

    Put your code in some directory **outside** of the document root, such as
    :file:`/home/mycode`.

Let's look at what :djadmin:`startproject` created::

    mysite/
        manage.py
        mysite/
            __init__.py
            settings.py
            urls.py
            wsgi.py

These files are:

* The outer :file:`mysite/` root directory is just a container for your
  project. Its name doesn't matter to Django; you can rename it to anything
  you like.

* :file:`manage.py`: A command-line utility that lets you interact with this
  Django project in various ways. You can read all the details about
  :file:`manage.py` in :doc:`/ref/django-admin`.

* The inner :file:`mysite/` directory is the actual Python package for your
  project. Its name is the Python package name you'll need to use to import
  anything inside it (e.g. ``mysite.urls``).

* :file:`mysite/__init__.py`: An empty file that tells Python that this
  directory should be considered a Python package. (Read `more about
  packages`_ in the official Python docs if you're a Python beginner.)

* :file:`mysite/settings.py`: Settings/configuration for this Django
  project.  :doc:`/topics/settings` will tell you all about how settings
  work.

* :file:`mysite/urls.py`: The URL declarations for this Django project; a
  "table of contents" of your Django-powered site. You can read more about
  URLs in :doc:`/topics/http/urls`.

* :file:`mysite/wsgi.py`: An entry-point for WSGI-compatible web servers to
  serve your project. See :doc:`/howto/deployment/wsgi/index` for more details.

.. _more about packages: https://docs.python.org/tutorial/modules.html#packages

Database setup
--------------

Now, edit :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.

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_psycopg2'``,
  ``'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, ``os.path.join(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`, :setting:`HOST` must be added.
For more details, see the reference documentation for :setting:`DATABASES`.

.. note::

    If you're using PostgreSQL or MySQL, make sure you've created a database by
    this point. Do that with "``CREATE DATABASE database_name;``" within your
    database's interactive prompt.

    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 in :doc:`part 2
  of this tutorial </intro/tutorial02>`.

* :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 makes 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:

.. code-block:: bash

    $ 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;`` (MySQL), or
``.schema`` (SQLite) 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`.

The development server
----------------------

Let's verify your Django project works. Change into the outer :file:`mysite` directory, if
you haven't already, and run the following commands:

.. code-block:: bash

   $ python manage.py runserver

You'll see the following output on the command line:

.. parsed-literal::

    Performing system checks...

    0 errors found
    |today| - 15:50:53
    Django version |version|, using settings 'mysite.settings'
    Starting development server at http://127.0.0.1:8000/
    Quit the server with CONTROL-C.

You've started the Django development server, a lightweight Web server written
purely in Python. We've included this with Django so you can develop things
rapidly, without having to deal with configuring a production server -- such as
Apache -- until you're ready for production.

Now's a good time to note: **don't** use this server in anything resembling a
production environment. It's intended only for use while developing. (We're in
the business of making Web frameworks, not Web servers.)

Now that the server's running, visit http://127.0.0.1:8000/ with your Web
browser. You'll see a "Welcome to Django" page, in pleasant, light-blue pastel.
It worked!

.. admonition:: Changing the port

    By default, the :djadmin:`runserver` command starts the development server
    on the internal IP at port 8000.

    If you want to change the server's port, pass
    it as a command-line argument. For instance, this command starts the server
    on port 8080:

    .. code-block:: bash

        $ python manage.py runserver 8080

    If you want to change the server's IP, pass it along with the port. So to
    listen on all public IPs (useful if you want to show off your work on other
    computers), use:

    .. code-block:: bash

        $ python manage.py runserver 0.0.0.0:8000

    Full docs for the development server can be found in the
    :djadmin:`runserver` reference.

.. admonition:: Automatic reloading of :djadmin:`runserver`

    The development server automatically reloads Python code for each request
    as needed. You don't need to restart the server for code changes to take
    effect. However, some actions like adding files don't trigger a restart,
    so you'll have to restart the server in these cases.

.. _creating-models:

Creating models
===============

Now that your environment -- a "project" -- is set up, you're set to start
doing work.

Each application you write in Django consists of a Python package that follows
a certain convention. Django comes with a utility that automatically generates
the basic directory structure of an app, so you can focus on writing code
rather than creating directories.

.. admonition:: Projects vs. apps

    What's the difference between a project and an app? An app is a Web
    application that does something -- e.g., a Weblog system, a database of
    public records or a simple poll app. A project is a collection of
    configuration and apps for a particular Web site. A project can contain
    multiple apps. An app can be in multiple projects.

Your apps can live anywhere on your `Python path`_. In this tutorial, we'll
create our poll app right next to your :file:`manage.py` file so that it can be
imported as its own top-level module, rather than a submodule of ``mysite``.

To create your app, make sure you're in the same directory as :file:`manage.py`
and type this command:

.. code-block:: bash

    $ python manage.py startapp polls

That'll create a directory :file:`polls`, which is laid out like this::

    polls/
        __init__.py
        admin.py
        migrations/
            __init__.py
        models.py
        tests.py
        views.py

This directory structure will house the poll application.

The first step in writing a database Web app in Django is to define your models
-- essentially, your database layout, with additional metadata.

.. admonition:: Philosophy

   A model is the single, definitive source of truth 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 just a
   history that Django can roll through to update your database schema to
   match your current models.

In our simple 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 simple Python classes. Edit the
:file:`polls/models.py` file so it looks like this:

.. snippet::
    :filename: 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)
        choice_text = models.CharField(max_length=200)
        votes = models.IntegerField(default=0)

The code is straightforward. 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.

.. _`Python path`: https://docs.python.org/tutorial/modules.html#the-module-search-path

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.

Edit the :file:`mysite/settings.py` file again, and change the
:setting:`INSTALLED_APPS` setting to include the string ``'polls'``. So it'll
look like this:

.. snippet::
    :filename: mysite/settings.py

    INSTALLED_APPS = [
        'django.contrib.admin',
        'django.contrib.auth',
        'django.contrib.contenttypes',
        'django.contrib.sessions',
        'django.contrib.messages',
        'django.contrib.staticfiles',
        'polls',
    ]

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

.. code-block:: bash

    $ python manage.py makemigrations polls

You should see something similar to the following:

.. code-block:: text

    Migrations for 'polls':
      0001_initial.py:
        - Create model Question
        - Create model Choice
        - Add field question to 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 just 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:

.. code-block:: bash

    $ 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 TABLE polls_question (
        "id" serial NOT NULL PRIMARY KEY,
        "question_text" varchar(200) NOT NULL,
        "pub_date" timestamp with time zone NOT NULL
    );

    CREATE TABLE polls_choice (
        "id" serial NOT NULL PRIMARY KEY,
        "question_id" integer NOT NULL,
        "choice_text" varchar(200) NOT NULL,
        "votes" integer NOT NULL
    );

    CREATE INDEX polls_choice_7aa0f6ee ON "polls_choice" ("question_id");

    ALTER TABLE "polls_choice"
      ADD CONSTRAINT polls_choice_question_id_246c99a640fbbd72_fk_polls_question_id
        FOREIGN KEY ("question_id")
        REFERENCES "polls_question" ("id")
        DEFERRABLE INITIALLY DEFERRED;
    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; that's just 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), ``serial`` (PostgreSQL), or ``integer
  primary key autoincrement`` (SQLite) are handled for you automatically. Same
  goes for 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 - it just 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:

.. code-block:: bash

    $ python manage.py migrate
    Operations to perform:
      Apply all migrations: admin, contenttypes, polls, auth, 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 there's 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 useable 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:

.. code-block:: bash

    $ python manage.py shell

We're using this instead of simply typing "python", because :file:`manage.py`
sets the ``DJANGO_SETTINGS_MODULE`` environment variable, which gives Django
the Python import path to your :file:`mysite/settings.py` file.

.. admonition:: Bypassing manage.py

    If you'd rather not use :file:`manage.py`, no problem. Just set the
    :envvar:`DJANGO_SETTINGS_MODULE` environment variable to
    ``mysite.settings``, start a plain Python shell, and set up Django:

    .. code-block:: pycon

        >>> import django
        >>> django.setup()

    If this raises an :exc:`AttributeError`, you're probably using
    a version of Django that doesn't match this tutorial version. You'll want
    to either switch to the older tutorial or the newer Django version.

    You must run ``python`` from the same directory :file:`manage.py` is in,
    or ensure that directory is on the Python path, so that ``import mysite``
    works.

    For more information on all of this, see the :doc:`django-admin
    documentation </ref/django-admin>`.

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

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

    # No questions are in the system yet.
    >>> Question.objects.all()
    []

    # 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. Note that this might say "1L" instead of "1", depending
    # on which database you're using. That's no biggie; it just means your
    # database backend prefers to return integers as Python long integer
    # objects.
    >>> 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=<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()
    [<Question: Question object>]


Wait a minute. ``<Question: Question object>`` is, utterly, an unhelpful 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``:

.. snippet::
    :filename: polls/models.py

    from django.db import models

    class Question(models.Model):
        # ...
        def __str__(self):              # __unicode__ on Python 2
            return self.question_text

    class Choice(models.Model):
        # ...
        def __str__(self):              # __unicode__ on Python 2
            return self.choice_text

It's important to add :meth:`~django.db.models.Model.__str__` methods to your
models, not only for your own sanity when dealing with the interactive prompt,
but also because objects' representations are used throughout Django's
automatically-generated admin.

.. admonition:: ``__str__`` or ``__unicode__``?

    On Python 3, it's easy, just use
    :meth:`~django.db.models.Model.__str__`.

    On Python 2, you should define :meth:`~django.db.models.Model.__unicode__`
    methods returning ``unicode`` values instead. Django models have a default
    :meth:`~django.db.models.Model.__str__` method that calls
    :meth:`~django.db.models.Model.__unicode__` and converts the result to a
    UTF-8 bytestring. This means that ``unicode(p)`` will return a Unicode
    string, and ``str(p)`` will return a bytestring, with characters encoded
    as UTF-8. Python does the opposite: ``object`` has a ``__unicode__``
    method that calls ``__str__`` and interprets the result as an ASCII
    bytestring. This difference can create confusion.

    If all of this is gibberish to you, just use Python 3.

Note these are normal Python methods. Let's add a custom method, just for
demonstration:

.. snippet::
    :filename: 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 Question, Choice

    # Make sure our __str__() addition worked.
    >>> Question.objects.all()
    [<Question: What's up?>]

    # Django provides a rich database lookup API that's entirely driven by
    # keyword arguments.
    >>> Question.objects.filter(id=1)
    [<Question: What's up?>]
    >>> Question.objects.filter(question_text__startswith='What')
    [<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()
    []

    # 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()
    [<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)
    [<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>`.

When you're comfortable with the API, read :doc:`part 2 of this tutorial
</intro/tutorial02>` to get Django's automatic admin working.