=====================================

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 polls.

We'll assume you have :doc:`Django installed </intro/install>` 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.

.. admonition:: Where to get help:

    If you're having trouble going through this tutorial, please post a message

    to `django-users`__ or drop by `#django on irc.freenode.net`__ to chat

    with other Django users who might be able to help.

__ http://groups.google.com/group/django-users

__ irc://irc.freenode.net/django

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 :term:`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.

.. admonition:: Script name may differ in distribution packages

   If you installed Django using a Linux distribution's package manager

   (e.g. apt-get or yum) ``django-admin.py`` may have been renamed to

   ``django-admin``. You may continue through this documentation by omitting

   ``.py`` from each command.

.. admonition:: Mac OS X permissions

   If you're using Mac OS X, you may see the message "permission denied" when

   you try to run ``django-admin.py startproject``. This is because, on

   Unix-based systems like OS X, a file must be marked as "executable" before it

   can be run as a program. To do this, open Terminal.app and navigate (using

   the ``cd`` command) to the directory where :doc:`django-admin.py

   </ref/django-admin>` is installed, then run the command

   ``chmod +x django-admin.py``.

.. note::

    You'll need to avoid naming projects after built-in Python or Django

    components. In particular, this means you should avoid using names like

    ``django`` (which will conflict with Django itself) or ``test`` (which

    conflicts with a built-in Python package).

:doc:`django-admin.py </ref/django-admin>` 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 :doc:`django-admin.py

</ref/django-admin>` from some place on your path, such as

:file:`/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

    :file:`/home/mycode`.

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

    mysite/

        __init__.py

        manage.py

        settings.py

        urls.py

These files are:

    * :file:`__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.)

    * :file:`manage.py`: A command-line utility that lets you interact with this

      Django project in various ways. You can read all the details about

      :file:`manage.py` in :doc:`/ref/django-admin`.

    * :file:`settings.py`: Settings/configuration for this Django project.

      :doc:`/topics/settings` will tell you all about how settings work.

    * :file:`urls.py`: The URL declarations for this Django project; a "table of

      contents" of your Django-powered site. You can read more about URLs in

      :doc:`/topics/http/urls`.

.. _more about packages: http://docs.python.org/tutorial/modules.html#packages

The development server

----------------------

Let's verify this worked. Change into the :file:`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 1.0, using settings 'mysite.settings'

    Development server is running at http://127.0.0.1:8000/

    Quit the server with CONTROL-C.

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 :djadmin:`runserver` command starts the development server

    on the internal IP at 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:

    .. code-block:: bash

        python manage.py runserver 8080

    If you want to change the server's IP, pass it along with the port. So to

    listen on all public IPs (useful if you want to show off your work on other

    computers), use:

    .. code-block:: bash

        python manage.py runserver 0.0.0.0:8000

    Full docs for the development server can be found in the

    :djadmin:`runserver` reference.

Database setup

--------------

Now, edit :file:`settings.py`. It's a normal Python module with

module-level variables representing Django settings. Change the

following keys in the :setting:`DATABASES` ``'default'`` item to match

your databases connection settings.

    * :setting:`ENGINE` -- Either

      ``'django.db.backends.postgresql_psycopg2'``,

      ``'django.db.backends.mysql'`` or

      ``'django.db.backends.sqlite3'``. Other backends are

      :setting:`also available <ENGINE>`.

    * :setting:`NAME` -- The name of your database. If you're using

      SQLite, the database will be a file on your computer; in that

      case, :setting:`NAME` should be the full absolute path,

      including filename, of that file. If the file doesn't exist, it

      will automatically be created when you synchronize the database

      for the first time (see below).

      When specifying the path, always use forward slashes, even on

      Windows (e.g. ``C:/homes/user/mysite/sqlite3.db``).

    * :setting:`USER` -- Your database username (not used for SQLite).

    * :setting:`PASSWORD` -- Your database password (not used for

      SQLite).

    * :setting:`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).

If you're new to databases, we recommend simply using SQLite (by

setting :setting:`ENGINE` to ``'django.db.backends.sqlite3'``). SQLite

is included as part of Python 2.5 and later, so you won't need to

install anything else.

.. 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.

    If you're using SQLite, you don't need to create anything beforehand - the

    database file will be created automatically when it is needed.

While you're editing :file:`settings.py`, take note of the

:setting:`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, :setting:`INSTALLED_APPS` contains the following apps, all of which

come with Django:

    * :mod:`django.contrib.auth` -- An authentication system.

    * :mod:`django.contrib.contenttypes` -- A framework for content types.

    * :mod:`django.contrib.sessions` -- A session framework.

    * :mod:`django.contrib.sites` -- A framework for managing multiple sites

      with one Django installation.

    * :mod:`django.contrib.messages` -- A messaging framework.

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:

.. code-block:: bash

    python manage.py syncdb

The :djadmin:`syncdb` command looks at the :setting:`INSTALLED_APPS` setting and

creates any necessary database tables according to the database settings in your

:file:`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

    :setting:`INSTALLED_APPS` before running :djadmin:`syncdb`. The

    :djadmin:`syncdb` command will only create tables for apps in

    :setting:`INSTALLED_APPS`.

.. _creating-models:

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.

Your apps can live anywhere on your `Python path`_. In this tutorial, we'll

create our poll app in the :file:`mysite` directory for simplicity.

To create your app, make sure you're in the :file:`mysite` directory and type

this command:

.. code-block:: bash

    python manage.py startapp polls

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

    polls/

        __init__.py

        models.py

        tests.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 :ref:`DRY Principle <dry>`. 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

:file:`polls/models.py` file so it looks like this::

    from django.db import models

    class Poll(models.Model):

        question = models.CharField(max_length=200)

        pub_date = models.DateTimeField('date published')

    class Choice(models.Model):

        poll = models.ForeignKey(Poll)

        choice = models.CharField(max_length=200)

        votes = models.IntegerField()

The code is straightforward. Each model is represented by a class that

subclasses :class:`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 :class:`~django.db.models.Field`

class -- e.g., :class:`~django.db.models.CharField` for character fields and

:class:`~django.db.models.DateTimeField` for datetimes. This tells Django what

type of data each field holds.

The name of each :class:`~django.db.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

:class:`~django.db.models.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 :class:`~django.db.models.Field` classes have required elements.

:class:`~django.db.models.CharField`, for example, requires that you give it a

:attr:`~django.db.models.Field.max_length`. That's used not only in the database

schema, but in validation, as we'll soon see.

Finally, note a relationship is defined, using

:class:`~django.db.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/tutorial/modules.html#the-module-search-path

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 :file:`settings.py` file again, and change the

:setting:`INSTALLED_APPS` setting to include the string ``'polls'``. So

it'll look like this::

    INSTALLED_APPS = (

        'django.contrib.auth',

        'django.contrib.contenttypes',

        'django.contrib.sessions',

        'django.contrib.sites',

        'polls'

    )

Now Django knows to include the ``polls`` app. Let's run another

command:

.. code-block:: bash

    python manage.py sql polls

You should see something similar to the following (the ``CREATE TABLE`` SQL

statements for the polls app):

.. code-block:: sql

    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:

    * The exact output will vary depending on the database you are using.

    * 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 :djadmin:`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:

    * :djadmin:`python manage.py validate <validate>` -- Checks for any errors

      in the construction of your models.

    * :djadmin:`python manage.py sqlcustom polls <sqlcustom>` -- Outputs any

      :ref:`custom SQL statements <initial-sql>` (such as table modifications or

      constraints) that are defined for the application.

    * :djadmin:`python manage.py sqlclear polls <sqlclear>` -- Outputs the

      necessary ``DROP TABLE`` statements for this app, according to which

      tables already exist in your database (if any).

    * :djadmin:`python manage.py sqlindexes polls <sqlindexes>` -- Outputs the

      ``CREATE INDEX`` statements for this app.

    * :djadmin:`python manage.py sqlall polls <sqlall>` -- A combination of all

      the SQL from the :djadmin:`sql`, :djadmin:`sqlcustom`, and

      :djadmin:`sqlindexes` commands.

Looking at the output of those commands can help you understand what's actually

happening under the hood.

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

.. code-block:: bash

    python manage.py syncdb

The :djadmin:`syncdb` command runs the sql from 'sqlall' on your database for

all apps in :setting:`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. :djadmin:`syncdb` can

be called as often as you like, and it will only ever create the tables that

don't exist.

Read the :doc:`django-admin.py documentation </ref/django-admin>` for full

information on what the ``manage.py`` utility can do.

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:

.. code-block:: bash

    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 ``polls`` on ``sys.path``. For flexibility, several pieces of

      Django refer to projects in Python dotted-path notation (e.g.

      ``'polls.models'``). In order for this to work, the ``polls``

      package has to be on ``sys.path``.

      We've already seen one example of this: the :setting:`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``

    and ``polls`` are at the root level on the Python path (i.e., ``import mysite``

    and ``import polls`` work) and set the ``DJANGO_SETTINGS_MODULE`` environment

    variable to ``mysite.settings``.

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

    documentation </ref/django-admin>`.

Once you're in the shell, explore the :doc:`database API </topics/db/queries>`::

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

    # No polls are in the system yet.

    >>> Poll.objects.all()

    []

    # Create a new Poll.

    >>> import datetime

    >>> p = Poll(question="What's up?", pub_date=datetime.datetime.now())