=====================================
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 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
    # 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' or 'mysql'. More coming soon.
* ``DATABASE_NAME`` -- The name of your database.
* ``DATABASE_USER`` -- Your database username.
* ``DATABASE_PASSWORD`` -- Your database password.
* ``DATABASE_HOST`` -- The host your database is on. Leave this as an
  empty string if your database server is on the same physical machine
  (localhost).

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

Once you've done that, you need to tell Django which settings module you're
currently using. Do that by setting an environment variable,
``DJANGO_SETTINGS_MODULE``. 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

Note this path is in Python package syntax. Your project has to be somewhere on
your `Python path`_ -- so that the Python statement ``import myproject.settings.main``
works. Throughout Django, you'll be referring to your projects and apps via
Python package syntax.

Then run the following command::

    django-admin.py init

If you don't see any errors, you know it worked. That command initialized your
database with Django's core database tables. If you're interested, run the
PostgreSQL or MySQL command-line client and type "\\dt" (PostgreSQL) or
"SHOW TABLES;" (MySQL) 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`: http://docs.python.org/tut/node8.html#SECTION008110000000000000000
.. _`Django 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

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.

   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', 'question', maxlength=200),
            meta.DateTimeField('pub_date', 'date published'),
        )

    class Choice(meta.Model):
        fields = (
            meta.ForeignKey(Poll),
            meta.CharField('choice', 'choice', maxlength=200),
            meta.IntegerField('votes', '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 argument is the field's human-readable name. That's used in a couple
of introspective parts of Django, and it doubles as documentation.

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.

   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) vs. ``serial`` (PostgreSQL) 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 ``DROP TABLE`` statements
  for this app.
* ``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 open the Python interactive shell, and 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(id=None, 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]

    # Make sure our custom method worked.
    >>> p = polls.get_object(id__exact=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(id__exact=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/