tutorial01.txt

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

By Adrian Holovaty <holovaty@gmail.com>

Let's learn by example.

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

It'll consist of two parts:

* A public site that lets people vote in polls and view poll results.
* An admin site that lets you add, change and delete polls behind the scenes.

We'll assume you have `Django installed`_ already.

.. _`Django installed`: http://www.djangoproject.com/documentation/install/

Initial setup
=============

If this is your first time using Django, you'll have to take care of some
initial setup.

Run the command ``django-admin.py startproject myproject``. That'll create a
``myproject`` directory in your current directory.

(``django-admin.py`` should be on your system path if you installed Django via
its setup.py utility. If it's not on your path, you can find it in
``site-packages/django/bin``; consider symlinking to it from some place
on your path, such as /usr/local/bin.)

A project is a collection of settings for an instance of Django -- including
database configuration, Django-specific options and application-specific
settings. Let's look at what ``startproject`` created::

    $ cd myproject/
    $ ls
    apps/  __init__.py  settings/
    $ ls settings/
    __init__.py  admin.py  main.py  urls/
    $ ls settings/urls/
    __init__.py  admin.py  main.py
    $ ls apps/
    __init__.py

First, edit ``myproject/settings/main.py``. It's a normal Python module with
module-level variables representing Django settings. Edit the file and change
these settings to match your database's connection parameters:

    * ``DATABASE_ENGINE`` -- Either 'postgresql', 'mysql' or 'sqlite3'.
      More coming soon.
    * ``DATABASE_NAME`` -- The name of your database, or the full path to
      the database file if using sqlite.
    * ``DATABASE_USER`` -- Your database username (not used for sqlite).
    * ``DATABASE_PASSWORD`` -- Your database password (not used for sqlite).
    * ``DATABASE_HOST`` -- The host your database is on. Leave this as an
      empty string if your database server is on the same physical machine
      (not used for sqlite).

.. admonition:: Note

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

    Also, note that MySQL and sqlite support is a recent development, and Django
    hasn't been comprehensively tested with either database. If you find any
    bugs in those bindings, please file them in `Django's ticket system`_ so we
    can fix them immediately.

Now, take a second to make sure ``myproject`` is on your Python path. You
can do this by copying ``myproject`` to Python's ``site-packages`` directory,
or you can do it by altering the ``PYTHONPATH`` environment variable. See the
`Python path documentation`_ for more information.

Run the following command::

    django-admin.py init --settings=myproject.settings.main

The ``django-admin.py`` utility generally needs to know which settings module
you're using. Here, we're doing that by specifying ``settings=`` on the command
line, but that can get tedious. If you don't want to type ``settings=`` each
time, you can set the ``DJANGO_SETTINGS_MODULE`` environment variable. Here's
how you do that in the Bash shell on Unix::

    export DJANGO_SETTINGS_MODULE=myproject.settings.main

On Windows, you'd use ``set`` instead::

    set DJANGO_SETTINGS_MODULE=myproject.settings.main

If you don't see any errors after running ``django-admin.py init``, you know it
worked. That command initialized your database with Django's core database
tables. 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.

Now you're set to start doing work. You won't have to take care of this boring
administrative stuff again.

.. _`Python path documentation`: http://docs.python.org/tut/node8.html#SECTION008110000000000000000
.. _Django's ticket system: http://code.djangoproject.com/report/1

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

Change into the ``myproject/apps`` directory and type this command::

    django-admin.py startapp polls

(From now on, this tutorial will leave out the ``--settings`` parameter and
will assume you've either set your ``DJANGO_SETTINGS_MODULE`` environment
variable or included the ``--settings`` option in your call to the command.)

That'll create a directory structure like this::

    polls/
        __init__.py
        models/
            __init__.py
            polls.py
        urls/
            __init__.py
            polls.py
        views/
            __init__.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 data about your
   data. It contains the essential fields and behaviors of the data you're
   storing. Django follows the `DRY Principle`_. The goal is to define your
   data model in one place and automatically derive things from it.

In our simple poll app, we'll create two models: polls and choices. A poll 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 poll.

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

    from django.core import meta

    class Poll(meta.Model):
        fields = (
            meta.CharField('question', maxlength=200),
            meta.DateTimeField('pub_date', 'date published'),
        )

    class Choice(meta.Model):
        fields = (
            meta.ForeignKey(Poll),
            meta.CharField('choice', maxlength=200),
            meta.IntegerField('votes'),
        )

The code is straightforward. Each model is represented by a class that
subclasses ``django.core.meta.Model``. Each model has a single class variable,
``fields``, which is a tuple of database fields in the model.

Each field is represented by an instance of a ``meta.*Field`` class -- e.g.,
``meta.CharField`` for character fields and ``meta.DateTimeField`` for
datetimes. This tells Django what type of data each field holds.

The first argument to each ``Field`` call 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.

The second, optional, argument is the field's 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 ``Poll.pub_date``.
For all other fields in this model, the field's machine-readable name will
suffice as its human-readable name.

Some ``meta.*Field`` classes have additional required elements.
``meta.CharField``, for example, requires that you give it a ``maxlength``.
That's used not only in the database schema, but in validation, as we'll soon
see.

Finally, note a relationship is defined, using ``meta.ForeignKey``. That tells
Django each Choice is related to a single Poll. Django supports all the common
database relationships: many-to-ones, many-to-manys and one-to-ones.

.. _DRY Principle: http://c2.com/cgi/wiki?DontRepeatYourself

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 Poll 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're not tied to a given
   Django installation.

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

    INSTALLED_APPS = (
        'myproject.apps.polls',
    )

(Don't forget the trailing comma because of Python's rules about single-value
tuples.)

Now Django knows myproject includes the polls app. Let's run another command::

    django-admin.py sql polls

You should see the following (the CREATE TABLE SQL statements for the polls app)::

    BEGIN;
    CREATE TABLE polls_polls (
        id serial NOT NULL PRIMARY KEY,
        question varchar(200) NOT NULL,
        pub_date timestamp with time zone NOT NULL
    );
    CREATE TABLE polls_choices (
        id serial NOT NULL PRIMARY KEY,
        poll_id integer NOT NULL REFERENCES polls_polls (id),
        choice varchar(200) NOT NULL,
        votes integer NOT NULL
    );
    COMMIT;

Note the following:

    * Table names are automatically generated by combining the name of the app
      (polls) with a plural version of the object name (polls and choices). (You
      can override this behavior.)

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

    * The foreign key relationship is made explicit by a ``REFERENCES`` statement.

    * 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`` (SQLite) are handled for you automatically. The author of
      this tutorial runs PostgreSQL, so the example output is in PostgreSQL
      syntax.

If you're interested, also run the following commands:

    * ``django-admin.py sqlinitialdata polls`` -- Outputs the initial-data
      inserts required for Django's admin framework.

    * ``django-admin.py sqlclear polls`` -- Outputs the necessary ``DROP
      TABLE`` statements for this app, according to which tables already exist
      in your database (if any).

    * ``django-admin.py sqlindexes polls`` -- Outputs the ``CREATE INDEX``
      statements for this app.

    * ``django-admin.py sqlall polls`` -- A combination of 'sql' and
      'sqlinitialdata'.

Looking at the output of those commands can help you understand what's actually
happening under the hood.

Now, run this command::

    django-admin.py install polls

That command automatically creates the database tables for the polls app.
Behind the scenes, all it does is take the output of
``django-admin.py sqlall polls`` and execute it in the database pointed-to by
your Django settings file.

Playing with the API
====================

Now, make sure your DJANGO_SETTINGS_MODULE environment variable is set (as
explained above), and open the Python interactive shell to play around with the
free Python API Django gives you::

    # Modules are dynamically created within django.models.
    # Their names are plural versions of the model class names.
    >>> from django.models.polls import polls, choices

    # No polls are in the system yet.
    >>> polls.get_list()
    []

    # Create a new Poll.
    >>> from datetime import datetime
    >>> p = polls.Poll(question="What's up?", pub_date=datetime.now())

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

    # Now it has an ID.
    >>> p.id
    1

    # Access database columns via Python attributes.
    >>> p.question
    "What's up?"
    >>> p.pub_date
    datetime.datetime(2005, 7, 15, 12, 00, 53)

    # Change values by changing the attributes, then calling save().
    >>> p.pub_date = datetime(2005, 4, 1, 0, 0)
    >>> p.save()

    # get_list() displays all the polls in the database.
    >>> polls.get_list()
    [<Poll object>]

Wait a minute. ``<Poll object>`` is, utterly, an unhelpful representation of
this object. Let's fix that by editing the polls model and adding a
``__repr__()`` method to both ``Poll`` and ``Choice``::

    class Poll(meta.Model):
        # ...
        def __repr__(self):
            return self.question

    class Choice(meta.Model):
        # ...
        def __repr__(self):
            return self.choice

It's important to add ``__repr__()`` 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.

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

    class Poll(meta.Model):
        # ...
        def was_published_today(self):
            return self.pub_date.date() == datetime.date.today()

Note ``import datetime`` wasn't necessary. Each model method has access to
a handful of commonly-used variables for convenience, including the
``datetime`` module from the Python standard library.

Let's jump back into the Python interactive shell::

    >>> from django.models.polls import polls, choices
    # Make sure our __repr__() addition worked.
    >>> polls.get_list()
    [What's up?]

    # Django provides a rich database lookup API that's entirely driven by
    # keyword arguments.
    >>> polls.get_object(id__exact=1)
    What's up
    >>> polls.get_object(question__startswith='What')
    What's up
    >>> polls.get_object(pub_date__year=2005)
    What's up
    >>> polls.get_object(id__exact=2)
    Traceback (most recent call last):
        ...
    PollDoesNotExist: Poll does not exist for {'id__exact': 2}
    >>> polls.get_list(question__startswith='What')
    [What's up]

    # 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 polls.get_object(id__exact=1).
    >>> polls.get_object(pk=1)
    What's up

    # Make sure our custom method worked.
    >>> p = polls.get_object(pk=1)
    >>> p.was_published_today()
    False

    # Give the Poll a couple of Choices. Each one of these method calls does an
    # INSERT statement behind the scenes and returns the new Choice object.
    >>> p = polls.get_object(pk=1)
    >>> p.add_choice(choice='Not much', votes=0)
    Not much
    >>> p.add_choice(choice='The sky', votes=0)
    The sky
    >>> c = p.add_choice(choice='Just hacking again', votes=0)

    # Choice objects have API access to their related Poll objects.
    >>> c.get_poll()
    What's up

    # And vice versa: Poll objects get access to Choice objects.
    >>> p.get_choice_list()
    [Not much, The sky, Just hacking again]
    >>> p.get_choice_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 poll whose pub_date is in 2005.
    >>> choices.get_list(poll__pub_date__year=2005)
    [Not much, The sky, Just hacking again]

    # Let's delete one of the choices. Use delete() for that.
    >>> c = p.get_choice(choice__startswith='Just hacking')
    >>> c.delete()

For full details on the database API, see our `Database API reference`_.

When you're comfortable with the API, read `part 2 of this tutorial`_ to get
Django's automatic admin working.

.. _Database API reference: http://www.djangoproject.com/documentation/db_api/
.. _part 2 of this tutorial: http://www.djangoproject.com/documentation/tutorial2/