=====================================
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.)

.. admonition:: Where should this code live?

    If your background is in PHP, 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
    ``/home/mycode``.

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
        manage.py
        settings.py
        urls.py

These files are:

    * ``manage.py``: A command-line utility that lets you interact with this
      Django project in various ways.
    * ``settings.py``: Settings/configuration for this Django project.
    * ``urls.py``: The URL declarations for this Django project; a "table of
      contents" of your Django-powered site.

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

Change into the ``myproject`` directory, if you haven't already, and run the
command ``python manage.py runserver``. You'll see the following output on the
command line::

    Validating models...
    0 errors found.

    Starting server on port 8000 with settings module 'myproject.settings'.
    Go to http://127.0.0.1:8000/ for Django.
    Quit the server with CONTROL-C (Unix) or CTRL-BREAK (Windows).

(If you get an error about ``DATABASE_ENGINE``, edit your ``settings.py`` file
to change the ``DATABASE_ENGINE`` setting to point to the correct database, and
make sure you have the right database libraries installed -- such as PostgreSQL's
psycopg or MySQL's MySQLdb.)

You've started the Django development server, a lightweight, pure-Python Web
server that builds on the BaseHTTPServer included in Python's standard library.
We've included this with Django so you can develop things rapidly, without
having to deal with configuring Apache until you're ready for production.

DON'T use this server in anything resembling a production environment. It's
intended only for use while developing.

.. admonition:: Changing the port

    By default, the ``runserver`` command starts the development server on port
    8000. If you want to change the server's port, pass it as a command-line
    argument::

        python manage.py runserver 8080

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!

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

Now, edit ``settings.py``. It's a normal Python module with module-level
variables representing Django settings. 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.

Run the following command to initialize your database with Django's core
database tables::

    python manage.py init

If you don't see any errors, it worked.

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:: About those database tables

    The tables created by ``manage.py init`` are for sessions, authentication
    and other features Django provides. The next release of Django will have
    a "lite" version of the ``init`` command that won't install any database
    tables if you don't want them.

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 that boring administrative stuff
again.)

Each application you write in Django 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.

.. 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.

In this tutorial, we'll create our poll app in the ``myproject`` 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.polls``.
Later in this tutorial, we'll discuss decoupling your apps for distribution.

To create your app, make sure you're in the ``myproject`` directory and type
this command::

    python manage.py startapp polls

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

    polls/
        __init__.py
        models/
            __init__.py
            polls.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 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.

.. _`Python path`: http://docs.python.org/tut/node8.html#SECTION008110000000000000000
.. _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 ``settings.py`` file again, and change the ``INSTALLED_APPS`` setting
to include the string ``'myproject.polls'``. So it'll look like this::

    INSTALLED_APPS = (
        'myproject.polls',
    )

(Don't forget the trailing comma, because of Python's rule about single-value
tuples: Without a trailing comma, Python wouldn't know this was a tuple.)

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

    python manage.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.)

    * 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. Same
      goes for quoting of field names -- e.g., using double quotes or single
      quotes. The author of this tutorial runs PostgreSQL, so the example
      output is inPostgreSQL syntax.

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

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

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

    * ``python manage.py sqlindexes polls`` -- Outputs the ``CREATE INDEX``
      statements for this app.

    * ``python manage.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::

    python manage.py install polls

Behind the scenes, all that command does is take the output of
``python manage.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 the
``manage.py`` utility can do.

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

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::

    python manage.py shell

We're using this instead of simply typing "python", because ``manage.py`` sets
up the project's environment for you. "Setting up the environment" involves two
things:

    * Putting ``myproject`` on ``sys.path``. For flexibility, several pieces of
      Django refer to projects in Python dotted-path notation (e.g.
      ``'myproject.polls.models'``). In order for this to work, the
      ``myproject`` package has to be on ``sys.path``.

      We've already seen one example of this: the ``INSTALLED_APPS`` setting is
      a list of packages in dotted-path notation.

    * Setting the ``DJANGO_SETTINGS_MODULE`` environment variable, which gives
      Django the path to your ``settings.py`` file.

.. admonition:: Bypassing manage.py

    If you'd rather not use ``manage.py``, no problem. Just make sure
    ``myproject`` is at the root level on the Python path (i.e.,
    ``import myproject`` works) and set the ``DJANGO_SETTINGS_MODULE``
    environment variable to ``myproject.settings``.

    For more information on all of this, see the `django-admin.py documentation`_.

Once you're in the shell, explore the database API::

    # 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. 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.
    >>> 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 by running
``python manage.py shell`` again::

    >>> 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?

    # Get the poll whose year is 2005. Of course, if you're going through this
    # tutorial in another year, change as appropriate.
    >>> 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/
