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

Let's learn by example.

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

It'll consist of two parts:

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

We'll assume you have `Django installed`_ already. You can tell Django is
installed by running the Python interactive interpreter and typing
``import django``. If that command runs successfully, with no errors, Django is
installed.

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

Creating a project
==================

If this is your first time using Django, you'll have to take care of some
initial setup. Namely, you'll need to auto-generate some code that establishes
a Django *project* -- a collection of settings for an instance of Django,
including database configuration, Django-specific options and
application-specific settings.

From the command line, ``cd`` into a directory where you'd like to store your
code, then run the command ``django-admin.py startproject mysite``. This
will create a ``mysite`` directory in your current directory.

(``django-admin.py`` should be on your system path if you installed Django via
``python setup.py``. If it's not on your path, you can find it in
``site-packages/django/bin``, where ``site-packages`` is a directory within
your Python installation. Consider symlinking to ``django-admin.py`` 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``.

Let's look at what ``startproject`` created::

    mysite/
        __init__.py
        manage.py
        settings.py
        urls.py

These files are:

    * ``__init__.py``: An empty file that tells Python that this directory
      should be considered a Python package. (Read `more about packages`_ in the
      official Python docs if you're a Python beginner.)
    * ``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.

.. _more about packages: http://docs.python.org/tut/node8.html#packages

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

Let's verify this worked. Change into the ``mysite`` 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.

    Django version 0.95, using settings 'mysite.settings'
    Development server is running at http://127.0.0.1:8000/
    Quit the server with CONTROL-C (Unix) or CTRL-BREAK (Windows).

You've started the Django development server, a lightweight Web server written
purely in Python. We've included this with Django so you can develop things
rapidly, without having to deal with configuring a production server -- such as
Apache -- until you're ready for production.

Now's a good time to note: DON'T use this server in anything resembling a
production environment. It's intended only for use while developing. (We're in
the business of making Web frameworks, not Web servers.)

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!

.. 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. For instance, this command starts the server on port 8080::

        python manage.py runserver 8080

    Full docs for the development server are at `django-admin documentation`_.

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

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

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

While you're editing ``settings.py``, take note of the ``INSTALLED_APPS``
setting towards the bottom of the file. That variable holds the names of all
Django applications that are activated in this Django instance. Apps can be
used in multiple projects, and you can package and distribute them for use
by others in their projects.

By default, ``INSTALLED_APPS`` contains the following apps, all of which come
with Django:

    * ``django.contrib.auth`` -- An authentication system.
    * ``django.contrib.contenttypes`` -- A framework for content types.
    * ``django.contrib.sessions`` -- A session framework.
    * ``django.contrib.sites`` -- A framework for managing multiple sites
      with one Django installation.

These applications are included by default as a convenience for the common
case.

Each of these applications makes use of at least one database table, though,
so we need to create the tables in the database before we can use them. To do
that, run the following command::

    python manage.py syncdb

The ``syncdb`` command looks at the ``INSTALLED_APPS`` setting and creates any
necessary database tables according to the database settings in your
``settings.py`` file. You'll see a message for each database table it creates,
and you'll get a prompt asking you if you'd like to create a superuser account
for the authentication system. Go ahead and do that.

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:: For the minimalists

    Like we said above, the default applications are included for the common
    case, but not everybody needs them. If you don't need any or all of them,
    feel free to comment-out or delete the appropriate line(s) from
    ``INSTALLED_APPS`` before running ``syncdb``. The ``syncdb`` command will
    only create tables for apps in ``INSTALLED_APPS``.

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

Now that your environment -- a "project" -- is set up, you're set to start
doing work.

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 ``mysite`` 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 ``mysite.polls``.
Later in this tutorial, we'll discuss decoupling your apps for distribution.

To create your app, make sure you're in the ``mysite`` 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.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.py`` file so it looks like this::

    from django.db import models

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

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

The code is straightforward. Each model is represented by a class that
subclasses ``django.db.models.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 ``models.*Field`` class -- e.g.,
``models.CharField`` for character fields and ``models.DateTimeField`` for
datetimes. This tells Django what type of data each field holds.

The name of each ``models.*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 ``Field`` classes have required elements. ``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 ``models.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 ``'mysite.polls'``. So it'll look like this::

    INSTALLED_APPS = (
        'django.contrib.auth',
        'django.contrib.contenttypes',
        'django.contrib.sessions',
        'django.contrib.sites',
        'mysite.polls'
    )

Now Django knows ``mysite`` 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_poll" (
        "id" serial NOT NULL PRIMARY KEY,
        "question" varchar(200) NOT NULL,
        "pub_date" timestamp with time zone NOT NULL
    );
    CREATE TABLE "polls_choice" (
        "id" serial NOT NULL PRIMARY KEY,
        "poll_id" integer NOT NULL REFERENCES "polls_poll" ("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``) and the lowercase name of the model -- ``poll`` and
      ``choice``. (You can override this behavior.)

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

    * By convention, Django appends ``"_id"`` to the foreign key field name.
      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 in PostgreSQL syntax.

    * The `sql` command doesn't actually run the SQL in your database - it just
      prints it to the screen so that you can see what SQL Django thinks is required.
      If you wanted to, you could copy and paste this SQL into your database prompt.
      However, as we will see shortly, Django provides an easier way of committing
      the SQL to the database.

If you're interested, also run the following commands:
    * ``python manage.py validate polls`` -- Checks for any errors in the
      construction of your models.

    * ``python manage.py sqlinitialdata polls`` -- Outputs any initial data
      required for Django's admin framework and your models.

    * ``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 all the SQL from
      the 'sql', 'sqlinitialdata', and 'sqlindexes' commands.

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

Now, run ``syncdb`` again to create those model tables in your database::

    python manage.py syncdb

The ``syncdb`` command runs the sql from 'sqlall' on your database for all apps
in ``INSTALLED_APPS`` that don't already exist in your database. This creates
all the tables, initial data and indexes for any apps you have added to your
project since the last time you ran syncdb. ``syncdb`` can be called as often
as you like, and it will only ever create the tables that don't exist.

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 ``mysite`` on ``sys.path``. For flexibility, several pieces of
      Django refer to projects in Python dotted-path notation (e.g.
      ``'mysite.polls.models'``). In order for this to work, the
      ``mysite`` 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
    ``mysite`` is at the root level on the Python path (i.e.,
    ``import mysite`` works) and set the ``DJANGO_SETTINGS_MODULE``
    environment variable to ``mysite.settings``.

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

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

    # Import the model classes we just wrote.
    >>> from mysite.polls.models import Poll, Choice

    # No polls are in the system yet.
    >>> Poll.objects.all()
    []

    # Create a new Poll.
    >>> from datetime import datetime
    >>> p = 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()

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


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

    class Poll(models.Model):
        # ...
        def __str__(self):
            return self.question

    class Choice(models.Model):
        # ...
        def __str__(self):
            return self.choice

It's important to add ``__str__()`` 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::

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

Note the addition of ``import datetime`` to reference Python's standard
``datetime`` module.

Let's jump back into the Python interactive shell by running
``python manage.py shell`` again::

    >>> from mysite.polls.models import Poll, Choice

    # Make sure our __str__() addition worked.
    >>> Poll.objects.all()
    [<Poll: What's up?>]

    # Django provides a rich database lookup API that's entirely driven by
    # keyword arguments.
    >>> Poll.objects.filter(id=1)
    [<Poll: What's up?>]
    >>> Poll.objects.filter(question__startswith='What')
    [<Poll: 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.
    >>> Poll.objects.get(pub_date__year=2005)
    <Poll: What's up?>

    >>> Poll.objects.get(id=2)
    Traceback (most recent call last):
        ...
    DoesNotExist: Poll matching query does not exist.

    # 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 Poll.objects.get(id=1).
    >>> Poll.objects.get(pk=1)
    <Poll: What's up?>

    # Make sure our custom method worked.
    >>> p = Poll.objects.get(pk=1)
    >>> p.was_published_today()
    False

    # Give the Poll a couple of Choices. The create call constructs a new
    # choice object, does the INSERT statement, adds the choice to the set
    # of available choices and returns the new Choice object.
    >>> p = Poll.objects.get(pk=1)
    >>> p.choice_set.create(choice='Not much', votes=0)
    <Choice: Not much>
    >>> p.choice_set.create(choice='The sky', votes=0)
    <Choice: The sky>
    >>> c = p.choice_set.create(choice='Just hacking again', votes=0)

    # Choice objects have API access to their related Poll objects.
    >>> c.poll
    <Poll: What's up?>

    # And vice versa: Poll objects get access to Choice objects.
    >>> p.choice_set.all()
    [<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]
    >>> p.choice_set.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.
    >>> Choice.objects.filter(poll__pub_date__year=2005)
    [<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]

    # Let's delete one of the choices. Use delete() for that.
    >>> c = p.choice_set.filter(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/