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=====================================
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Writing your first Django app, part 1
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=====================================
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Let's learn by example.
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Throughout this tutorial, we'll walk you through the creation of a basic
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It'll consist of two parts:
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* A public site that lets people view polls and vote in them.
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* An admin site that lets you add, change and delete poll.
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We'll assume you have `Django installed`_ already. You can tell Django is
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installed by running the Python interactive interpreter and typing
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``import django``. If that command runs successfully, with no errors, Django is
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.. _`Django installed`: ../install/
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.. admonition:: Where to get help:
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If you're having trouble going through this tutorial, please post a message
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to `django-users`_ or drop by `#django`_ on ``irc.freenode.net`` and we'll
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.. _django-users: http://groups.google.com/group/django-users
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.. _#django: irc://irc.freenode.net/django
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If this is your first time using Django, you'll have to take care of some
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initial setup. Namely, you'll need to auto-generate some code that establishes
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a Django *project* -- a collection of settings for an instance of Django,
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including database configuration, Django-specific options and
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application-specific settings.
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From the command line, ``cd`` into a directory where you'd like to store your
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code, then run the command ``django-admin.py startproject mysite``. This
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will create a ``mysite`` directory in your current directory.
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You'll need to avoid naming projects after built-in Python or Django
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components. In particular, this means you should avoid using names like
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``django`` (which will conflict with Django itself) or ``site`` (which
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conflicts with a built-in Python package).
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(``django-admin.py`` should be on your system path if you installed Django via
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``python setup.py``. If it's not on your path, you can find it in
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``site-packages/django/bin``, where ``site-packages`` is a directory within
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your Python installation. Consider symlinking to ``django-admin.py`` from some
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place on your path, such as ``/usr/local/bin``.)
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.. admonition:: Where should this code live?
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If your background is in PHP, you're probably used to putting code under the
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Web server's document root (in a place such as ``/var/www``). With Django,
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you don't do that. It's not a good idea to put any of this Python code within
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your Web server's document root, because it risks the possibility that
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people may be able to view your code over the Web. That's not good for
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Put your code in some directory **outside** of the document root, such as
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Let's look at what ``startproject`` created::
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* ``__init__.py``: An empty file that tells Python that this directory
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should be considered a Python package. (Read `more about packages`_ in the
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official Python docs if you're a Python beginner.)
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* ``manage.py``: A command-line utility that lets you interact with this
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Django project in various ways.
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* ``settings.py``: Settings/configuration for this Django project.
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* ``urls.py``: The URL declarations for this Django project; a "table of
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contents" of your Django-powered site.
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.. _more about packages: http://docs.python.org/tut/node8.html#packages
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The development server
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----------------------
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Let's verify this worked. Change into the ``mysite`` directory, if you
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haven't already, and run the command ``python manage.py runserver``. You'll see
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the following output on the command line::
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Django version 0.95, using settings 'mysite.settings'
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Development server is running at http://127.0.0.1:8000/
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Quit the server with CONTROL-C (Unix) or CTRL-BREAK (Windows).
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You've started the Django development server, a lightweight Web server written
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purely in Python. We've included this with Django so you can develop things
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rapidly, without having to deal with configuring a production server -- such as
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Apache -- until you're ready for production.
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Now's a good time to note: DON'T use this server in anything resembling a
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production environment. It's intended only for use while developing. (We're in
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the business of making Web frameworks, not Web servers.)
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Now that the server's running, visit http://127.0.0.1:8000/ with your Web
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browser. You'll see a "Welcome to Django" page, in pleasant, light-blue pastel.
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.. admonition:: Changing the port
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By default, the ``runserver`` command starts the development server on port
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8000. If you want to change the server's port, pass it as a command-line
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argument. For instance, this command starts the server on port 8080::
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python manage.py runserver 8080
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Full docs for the development server are at `django-admin documentation`_.
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.. _django-admin documentation: ../django_admin/
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Now, edit ``settings.py``. It's a normal Python module with module-level
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variables representing Django settings. Change these settings to match your
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database's connection parameters:
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* ``DATABASE_ENGINE`` -- Either 'postgresql', 'mysql' or 'sqlite3'.
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* ``DATABASE_NAME`` -- The name of your database, or the full (absolute)
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path to the database file if you're using SQLite.
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* ``DATABASE_USER`` -- Your database username (not used for SQLite).
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* ``DATABASE_PASSWORD`` -- Your database password (not used for SQLite).
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* ``DATABASE_HOST`` -- The host your database is on. Leave this as an
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empty string if your database server is on the same physical machine
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(not used for SQLite).
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If you're using PostgreSQL or MySQL, make sure you've created a database by
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this point. Do that with "``CREATE DATABASE database_name;``" within your
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database's interactive prompt.
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While you're editing ``settings.py``, take note of the ``INSTALLED_APPS``
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setting towards the bottom of the file. That variable holds the names of all
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Django applications that are activated in this Django instance. Apps can be
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used in multiple projects, and you can package and distribute them for use
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by others in their projects.
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By default, ``INSTALLED_APPS`` contains the following apps, all of which come
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* ``django.contrib.auth`` -- An authentication system.
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* ``django.contrib.contenttypes`` -- A framework for content types.
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* ``django.contrib.sessions`` -- A session framework.
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* ``django.contrib.sites`` -- A framework for managing multiple sites
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with one Django installation.
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These applications are included by default as a convenience for the common
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Each of these applications makes use of at least one database table, though,
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so we need to create the tables in the database before we can use them. To do
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that, run the following command::
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python manage.py syncdb
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The ``syncdb`` command looks at the ``INSTALLED_APPS`` setting and creates any
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necessary database tables according to the database settings in your
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``settings.py`` file. You'll see a message for each database table it creates,
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and you'll get a prompt asking you if you'd like to create a superuser account
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for the authentication system. Go ahead and do that.
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If you're interested, run the command-line client for your database and type
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``\dt`` (PostgreSQL), ``SHOW TABLES;`` (MySQL), or ``.schema`` (SQLite) to
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display the tables Django created.
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.. admonition:: For the minimalists
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Like we said above, the default applications are included for the common
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case, but not everybody needs them. If you don't need any or all of them,
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feel free to comment-out or delete the appropriate line(s) from
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``INSTALLED_APPS`` before running ``syncdb``. The ``syncdb`` command will
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only create tables for apps in ``INSTALLED_APPS``.
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Now that your environment -- a "project" -- is set up, you're set to start
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Each application you write in Django consists of a Python package, somewhere
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on your `Python path`_, that follows a certain convention. Django comes with a
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utility that automatically generates the basic directory structure of an app,
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so you can focus on writing code rather than creating directories.
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.. admonition:: Projects vs. apps
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What's the difference between a project and an app? An app is a Web
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application that does something -- e.g., a weblog system, a database of
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public records or a simple poll app. A project is a collection of
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configuration and apps for a particular Web site. A project can contain
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multiple apps. An app can be in multiple projects.
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In this tutorial, we'll create our poll app in the ``mysite`` directory,
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for simplicity. As a consequence, the app will be coupled to the project --
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that is, Python code within the poll app will refer to ``mysite.polls``.
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Later in this tutorial, we'll discuss decoupling your apps for distribution.
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To create your app, make sure you're in the ``mysite`` directory and type
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python manage.py startapp polls
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That'll create a directory ``polls``, which is laid out like this::
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This directory structure will house the poll application.
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The first step in writing a database Web app in Django is to define your models
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.. admonition:: Philosophy
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A model is the single, definitive source of data about your
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data. It contains the essential fields and behaviors of the data you're
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storing. Django follows the `DRY Principle`_. The goal is to define your
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data model in one place and automatically derive things from it.
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In our simple poll app, we'll create two models: polls and choices. A poll has
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a question and a publication date. A choice has two fields: the text of the
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choice and a vote tally. Each choice is associated with a poll.
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These concepts are represented by simple Python classes. Edit the
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``polls/models.py`` file so it looks like this::
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from django.db import models
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class Poll(models.Model):
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question = models.CharField(maxlength=200)
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pub_date = models.DateTimeField('date published')
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class Choice(models.Model):
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poll = models.ForeignKey(Poll)
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choice = models.CharField(maxlength=200)
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votes = models.IntegerField()
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The code is straightforward. Each model is represented by a class that
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subclasses ``django.db.models.Model``. Each model has a number of class
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variables, each of which represents a database field in the model.
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Each field is represented by an instance of a ``models.*Field`` class -- e.g.,
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``models.CharField`` for character fields and ``models.DateTimeField`` for
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datetimes. This tells Django what type of data each field holds.
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The name of each ``models.*Field`` instance (e.g. ``question`` or ``pub_date`` )
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is the field's name, in machine-friendly format. You'll use this value in your
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Python code, and your database will use it as the column name.
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You can use an optional first positional argument to a ``Field`` to designate a
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human-readable name. That's used in a couple of introspective parts of Django,
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and it doubles as documentation. If this field isn't provided, Django will use
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the machine-readable name. In this example, we've only defined a human-readable
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name for ``Poll.pub_date``. For all other fields in this model, the field's
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machine-readable name will suffice as its human-readable name.
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Some ``Field`` classes have required elements. ``CharField``, for example,
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requires that you give it a ``maxlength``. That's used not only in the database
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schema, but in validation, as we'll soon see.
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Finally, note a relationship is defined, using ``models.ForeignKey``. That tells
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Django each Choice is related to a single Poll. Django supports all the common
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database relationships: many-to-ones, many-to-manys and one-to-ones.
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.. _`Python path`: http://docs.python.org/tut/node8.html#SECTION008110000000000000000
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.. _DRY Principle: http://c2.com/cgi/wiki?DontRepeatYourself
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That small bit of model code gives Django a lot of information. With it, Django
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* Create a database schema (``CREATE TABLE`` statements) for this app.
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* Create a Python database-access API for accessing Poll and Choice objects.
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But first we need to tell our project that the ``polls`` app is installed.
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.. admonition:: Philosophy
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Django apps are "pluggable": You can use an app in multiple projects, and
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you can distribute apps, because they don't have to be tied to a given
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Edit the ``settings.py`` file again, and change the ``INSTALLED_APPS`` setting
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to include the string ``'mysite.polls'``. So it'll look like this::
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'django.contrib.auth',
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'django.contrib.contenttypes',
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'django.contrib.sessions',
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'django.contrib.sites',
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Now Django knows ``mysite`` includes the ``polls`` app. Let's run another command::
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python manage.py sql polls
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You should see the following (the CREATE TABLE SQL statements for the polls app)::
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CREATE TABLE "polls_poll" (
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"id" serial NOT NULL PRIMARY KEY,
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"question" varchar(200) NOT NULL,
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"pub_date" timestamp with time zone NOT NULL
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CREATE TABLE "polls_choice" (
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"id" serial NOT NULL PRIMARY KEY,
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"poll_id" integer NOT NULL REFERENCES "polls_poll" ("id"),
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"choice" varchar(200) NOT NULL,
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"votes" integer NOT NULL
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* Table names are automatically generated by combining the name of the app
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(``polls``) and the lowercase name of the model -- ``poll`` and
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``choice``. (You can override this behavior.)
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* Primary keys (IDs) are added automatically. (You can override this, too.)
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* By convention, Django appends ``"_id"`` to the foreign key field name.
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Yes, you can override this, as well.
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* The foreign key relationship is made explicit by a ``REFERENCES`` statement.
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* It's tailored to the database you're using, so database-specific field
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types such as ``auto_increment`` (MySQL), ``serial`` (PostgreSQL), or
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``integer primary key`` (SQLite) are handled for you automatically. Same
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goes for quoting of field names -- e.g., using double quotes or single
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quotes. The author of this tutorial runs PostgreSQL, so the example
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output is in PostgreSQL syntax.
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* The `sql` command doesn't actually run the SQL in your database - it just
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prints it to the screen so that you can see what SQL Django thinks is required.
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If you wanted to, you could copy and paste this SQL into your database prompt.
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However, as we will see shortly, Django provides an easier way of committing
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the SQL to the database.
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If you're interested, also run the following commands:
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* ``python manage.py validate polls`` -- Checks for any errors in the
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construction of your models.
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* ``python manage.py sqlinitialdata polls`` -- Outputs any initial data
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required for Django's admin framework and your models.
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* ``python manage.py sqlclear polls`` -- Outputs the necessary ``DROP
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TABLE`` statements for this app, according to which tables already exist
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in your database (if any).
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* ``python manage.py sqlindexes polls`` -- Outputs the ``CREATE INDEX``
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statements for this app.
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* ``python manage.py sqlall polls`` -- A combination of all the SQL from
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the 'sql', 'sqlinitialdata', and 'sqlindexes' commands.
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Looking at the output of those commands can help you understand what's actually
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happening under the hood.
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Now, run ``syncdb`` again to create those model tables in your database::
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python manage.py syncdb
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The ``syncdb`` command runs the sql from 'sqlall' on your database for all apps
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in ``INSTALLED_APPS`` that don't already exist in your database. This creates
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all the tables, initial data and indexes for any apps you have added to your
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project since the last time you ran syncdb. ``syncdb`` can be called as often
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as you like, and it will only ever create the tables that don't exist.
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Read the `django-admin.py documentation`_ for full information on what the
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``manage.py`` utility can do.
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.. _django-admin.py documentation: ../django_admin/
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Now, let's hop into the interactive Python shell and play around with the free
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API Django gives you. To invoke the Python shell, use this command::
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python manage.py shell
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We're using this instead of simply typing "python", because ``manage.py`` sets
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up the project's environment for you. "Setting up the environment" involves two
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* Putting ``mysite`` on ``sys.path``. For flexibility, several pieces of
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Django refer to projects in Python dotted-path notation (e.g.
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``'mysite.polls.models'``). In order for this to work, the
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``mysite`` package has to be on ``sys.path``.
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We've already seen one example of this: the ``INSTALLED_APPS`` setting is
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a list of packages in dotted-path notation.
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* Setting the ``DJANGO_SETTINGS_MODULE`` environment variable, which gives
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Django the path to your ``settings.py`` file.
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.. admonition:: Bypassing manage.py
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If you'd rather not use ``manage.py``, no problem. Just make sure
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``mysite`` is at the root level on the Python path (i.e.,
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``import mysite`` works) and set the ``DJANGO_SETTINGS_MODULE``
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environment variable to ``mysite.settings``.
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For more information on all of this, see the `django-admin.py documentation`_.
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Once you're in the shell, explore the database API::
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# Import the model classes we just wrote.
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>>> from mysite.polls.models import Poll, Choice
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# No polls are in the system yet.
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>>> Poll.objects.all()
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>>> from datetime import datetime
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>>> p = Poll(question="What's up?", pub_date=datetime.now())
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# Save the object into the database. You have to call save() explicitly.
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# Now it has an ID. Note that this might say "1L" instead of "1", depending
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# on which database you're using. That's no biggie; it just means your
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# database backend prefers to return integers as Python long integer
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# Access database columns via Python attributes.
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datetime.datetime(2005, 7, 15, 12, 00, 53)
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# Change values by changing the attributes, then calling save().
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>>> p.pub_date = datetime(2005, 4, 1, 0, 0)
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# objects.all() displays all the polls in the database.
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>>> Poll.objects.all()
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[<Poll: Poll object>]
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Wait a minute. ``<Poll: Poll object>`` is, utterly, an unhelpful
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representation of this object. Let's fix that by editing the polls model (in
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the ``polls/models.py`` file) and adding a ``__str__()`` method to both
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``Poll`` and ``Choice``::
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class Poll(models.Model):
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class Choice(models.Model):
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It's important to add ``__str__()`` methods to your models, not only for your
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own sanity when dealing with the interactive prompt, but also because objects'
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representations are used throughout Django's automatically-generated admin.
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Note these are normal Python methods. Let's add a custom method, just for
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class Poll(models.Model):
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def was_published_today(self):
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return self.pub_date.date() == datetime.date.today()
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Note the addition of ``import datetime`` to reference Python's standard
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Let's jump back into the Python interactive shell by running
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``python manage.py shell`` again::
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>>> from mysite.polls.models import Poll, Choice
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# Make sure our __str__() addition worked.
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>>> Poll.objects.all()
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# Django provides a rich database lookup API that's entirely driven by
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>>> Poll.objects.filter(id=1)
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>>> Poll.objects.filter(question__startswith='What')
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# Get the poll whose year is 2005. Of course, if you're going through this
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# tutorial in another year, change as appropriate.
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>>> Poll.objects.get(pub_date__year=2005)
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>>> Poll.objects.get(id=2)
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Traceback (most recent call last):
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DoesNotExist: Poll matching query does not exist.
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# Lookup by a primary key is the most common case, so Django provides a
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# shortcut for primary-key exact lookups.
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# The following is identical to Poll.objects.get(id=1).
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>>> Poll.objects.get(pk=1)
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# Make sure our custom method worked.
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>>> p = Poll.objects.get(pk=1)
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>>> p.was_published_today()
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# Give the Poll a couple of Choices. The create call constructs a new
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# choice object, does the INSERT statement, adds the choice to the set
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# of available choices and returns the new Choice object.
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>>> p = Poll.objects.get(pk=1)
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>>> p.choice_set.create(choice='Not much', votes=0)
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>>> p.choice_set.create(choice='The sky', votes=0)
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>>> c = p.choice_set.create(choice='Just hacking again', votes=0)
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# Choice objects have API access to their related Poll objects.
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# And vice versa: Poll objects get access to Choice objects.
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>>> p.choice_set.all()
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[<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]
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>>> p.choice_set.count()
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# The API automatically follows relationships as far as you need.
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# Use double underscores to separate relationships.
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# This works as many levels deep as you want. There's no limit.
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# Find all Choices for any poll whose pub_date is in 2005.
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>>> Choice.objects.filter(poll__pub_date__year=2005)
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[<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]
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# Let's delete one of the choices. Use delete() for that.
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>>> c = p.choice_set.filter(choice__startswith='Just hacking')
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For full details on the database API, see our `Database API reference`_.
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When you're comfortable with the API, read `part 2 of this tutorial`_ to get
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Django's automatic admin working.
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.. _Database API reference: ../db_api/
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.. _part 2 of this tutorial: ../tutorial2/
added
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docs/tutorial01.txt
