=====================================
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::

    myproject/
        __init__.py
        apps/
            __init__.py
        settings.py
        urls.py

First, edit ``myproject/settings.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 (absolute)
      path to the database file if you're 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.

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. If you opt to set the
``PYTHONPATH`` environment variable, note that you'll need to set it to the
*parent* directory of ``myproject``. (You can test this by typing
"import myproject" into the Python interactive prompt.)

Run the following command::

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

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

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

    set DJANGO_SETTINGS_MODULE=myproject.settings

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.

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

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

Now that your environment -- a "project" -- is set up, you're set to start
doing work. (You won't have to take care of this boring administrative stuff
again.)

Each application you write in Django -- e.g., a weblog system, a database of
public records or a simple poll app -- consists of a Python package, somewhere
on your Python path, 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.

In this tutorial, we'll create our poll app in the ``myproject/apps``
directory, for simplicity. As a consequence, the app will be coupled to the
project -- that is, Python code within the poll app will refer to
``myproject.apps.polls``. Later in this tutorial, we'll discuss decoupling
your apps for distribution.

To create your app, 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
        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.

These concepts are represented by simple Python classes. Edit the
``polls/models/polls.py`` file so it looks like this::

    from django.core import meta

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

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

The code is straightforward. Each model is represented by a class that
subclasses ``django.core.meta.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 ``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 name of each ``meta.*Field`` instance (e.g. ``question`` 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 ``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 ``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 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 don't have to be tied to
   a given Django installation.

Edit the myproject/settings.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

(Note that it doesn't matter which directory you're in when you run this command.)

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.)

    * Django appends ``"_id"`` to the foreign key field name, by convention.
      Yes, you can override this, as well.

    * 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 to create the database tables for the polls app
automatically::

    django-admin.py install polls

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

Read the `django-admin.py documentation`_ for full information on what this
utility can do.

.. _django-admin.py documentation: http://www.djangoproject.com/documentation/django_admin/

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
(in the ``polls/models/polls.py`` file) 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/