django/docs/tutorial01.txt

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Tutorial: Writing your first Django app
=======================================

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).

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``::

    export 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

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, a publication date and an expiration 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`_.

.. _Database API reference: http://www.djangoproject.com/documentation/db_api/

Coming soon
===========

The tutorial ends here for the time being. But check back within 48 hours for
the next installments:

* Using the dynamically-generated admin site
* Writing public-facing apps
* Using the cache framework
* Using the RSS framework