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Adrian Holovaty
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=======================================
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Tutorial: Writing your first Django app
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=======================================
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Let's learn by example.
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Throughout this documentation, we'll walk you through the creation of a simple
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Web poll application.
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It'll consist of two parts:
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* A public site that lets people vote in polls and view poll results.
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* An admin site that lets you add, change and delete polls behind the scenes.
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Initial setup
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=============
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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.
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Run the command ``django-admin.py startproject myproject``. That'll create a
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``myproject`` directory in your current directory.
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(``django-admin.py`` should be on your path if you installed Django via
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its setup.py utility. If it's not on your path, you can find it in
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``site-packages/django/bin``; consider symlinking to it from some place
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on your path, such as /usr/local/bin.)
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A project is a collection of settings for an instance of Django -- including
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database configuration, Django-specific options and application-specific
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settings. Let's look at what ``startproject`` created::
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$ cd myproject/
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$ ls
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apps/ __init__.py settings/
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$ ls settings/
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__init__.py admin.py main.py
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# ls apps/
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__init__.py
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First, edit ``myproject/settings/main.py``. It's a normal Python module with
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module-level variables representing Django settings. Edit the file and change
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these settings to match your database's connection parameters::
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* ``DATABASE_ENGINE`` -- Either 'postgresql' or 'mysql'. More coming soon.
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* ``DATABASE_NAME`` -- The name of your database.
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* ``DATABASE_USER`` -- Your database username.
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* ``DATABASE_PASSWORD`` -- Your database password.
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* ``DATABASE_HOST`` -- The host your database is on. Just leave this as an
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empty string if your database server is on the same physical machine
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(localhost).
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Once you've done that, you need to tell Django which settings module you're
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currently using. Do that by setting an environment variable,
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``DJANGO_SETTINGS_MODULE``::
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export DJANGO_SETTINGS_MODULE='myproject.settings.main'
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Note this path is in Python package syntax. Your project has to be somewhere on
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your `Python path`_ -- so that the Python statement ``import myproject.settings.main``
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works. Throughout Django, you'll be referring to your projects and apps via
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Python package syntax.
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Then run the following command:
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django-admin.py init
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If you don't see any errors, you know it worked. That command initialized your
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database with Django's core database tables. If you're interested, run the
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PostgreSQL or MySQL command-line client and type "\dt" (PostgreSQL) or
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"SHOW TABLES" (MySQL) to display the tables.
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Now you're set to start doing work. You won't have to take care of this boring
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administrative stuff again.
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:: _Python path: http://docs.python.org/tut/node8.html#SECTION008110000000000000000
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Creating models
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===============
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``cd`` into the ``myproject/apps`` directory and type this command:
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django-admin.py startapp polls
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That'll create a directory structure like this:
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polls/
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__init__.py
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models/
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__init__.py
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polls.py
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urls/
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__init__.py
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polls.py
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views/
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__init__.py
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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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-- essentially, your database layout, with additional metadata.
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PHILOSOPHY: 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, a publication date and an expiration date. A choice has two fields:
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the text of the choice and a vote tally. Each choice is associated with a poll.
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Edit the ``polls/models/polls.py`` file so that it looks like this:
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from django.core import meta
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class Poll(meta.Model):
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fields = (
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meta.CharField('question', 'question', maxlength=200),
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meta.DateTimeField('pub_date', 'date published'),
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)
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class Choice(meta.Model):
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fields = (
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meta.ForeignKey(Poll),
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meta.CharField('choice', 'choice', maxlength=200),
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meta.IntegerField('votes', 'votes'),
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)
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The code is straightforward. Each model is represented by a class that
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subclasses ``django.core.meta.Model``. Each model has a single class variable,
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``fields``, which is a tuple of database fields in the model.
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Each field is represented by an instance of a ``meta.*Field`` class -- e.g.,
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``meta.CharField`` for character fields and ``meta.DateTimeField`` for
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datetimes. This tells Django what type of data each field holds.
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The first argument to each ``Field`` call is the field's name, in
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machine-friendly format. You'll use this value in your Python code, and your
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database will use it as the column name.
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The second argument is the field's human-readable name. That's used in a couple
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of introspective parts of Django, and it doubles as documentation.
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Some ``meta.*Field`` classes have additional required elements.
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``meta.CharField``, for example, requires that you give it a ``maxlength``.
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That's used not only in the database schema, but in validation, as we'll soon
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see.
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Finally, note a relationship is defined, using ``meta.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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.. _DRY Principle: http://c2.com/cgi/wiki?DontRepeatYourself
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Activating models
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=================
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That small bit of model code gives Django a lot of information. With it, Django
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is able to:
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* Create a database schema (``CREATE TABLE`` statements) for this app.
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* Create a Python 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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PHILOSOPHY: Django apps are "pluggable": You can use an app in multiple
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projects, and you can distribute apps, because they're not tied to a given
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Django installation.
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Edit the myproject/settings/main.py file again, and change the ``INSTALLED_APPS``
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setting to include the string "myproject.apps.polls". So it'll look like this::
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INSTALLED_APPS = (
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'myproject.apps.polls',
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)
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(Don't forget the trailing comma because of Python's rules about single-value
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tuples.)
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Now Django knows myproject includes the polls app. Let's run another command:
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django-admin.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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BEGIN;
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CREATE TABLE polls_polls (
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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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);
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CREATE TABLE polls_choices (
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id serial NOT NULL PRIMARY KEY,
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poll_id integer NOT NULL REFERENCES polls_polls (id),
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choice varchar(200) NOT NULL,
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votes integer NOT NULL
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);
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COMMIT;
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Note the following:
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* Table names are automatically generated by combining the name of the app
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(polls) with a plural version of the object name (polls and choices).
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* Primary keys (IDs) are added automatically. (You can override this behavior.)
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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 types
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such as ``auto_increment`` (MySQL) vs. ``serial`` (PostgreSQL) are handled
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for you automatically. The author of this tutorial runs PostgreSQL, so the
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example output is in PostgreSQL syntax.
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If you're interested, also run the following commands:
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* ``django-admin.py sqlinitialdata polls`` -- Outputs the initial-data inserts
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required for Django's admin framework.
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* ``django-admin.py sqlclear polls`` -- Outputs the ``DROP TABLE`` statements
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for this app.
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* ``django-admin.py sqlindexes polls`` -- Outputs the ``CREATE INDEX``
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statements for this app.
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* ``django-admin.py sqlall polls`` -- A combination of 'sql' and
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'sqlinitialdata'.
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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 this command:
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django-admin.py install polls
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That command automatically creates the database tables for the polls app.
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Behind the scenes, all it does is take the output of
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``django-admin.py sqlall polls`` and execute it in the database pointed-to by
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your Django settings file.
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Playing with the API
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====================
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Now open the Python interactive shell, and play around with the free Python API
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Django gives you::
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# Modules are dynamically created within django.models.
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# Their names are plural versions of the model class names.
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>>> from django.models.polls import polls, choices
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# No polls are in the system yet.
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>>> polls.get_list()
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[]
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# Create a new Poll.
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>>> from datetime import datetime
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>>> p = polls.Poll(id=None, 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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>>> p.save()
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# Now it has an ID.
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>>> p.id
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1
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# Access database columns via Python attributes.
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>>> p.question
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"What's up?"
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>>> p.pub_date
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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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>>> p.save()
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# get_list() displays all the polls in the database.
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>>> polls.get_list()
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[<Poll object>]
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Wait a minute. ``<Poll object>`` is, utterly, an unhelpful representation of
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this object. Let's fix that by editing the polls model and adding a
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``__repr__()`` method to both ``Poll`` and ``Choice``::
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class Poll(meta.Model):
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# ...
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def __repr__(self):
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return self.question
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class Choice(meta.Model):
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# ...
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def __repr__(self):
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return self.choice
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It's important to add ``__repr__()`` 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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demonstration::
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class Poll(meta.Model):
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# ...
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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 ``import datetime`` wasn't necessary. Each model method has access to
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a handful of commonly-used variables for convenience, including the
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``datetime`` module from the Python standard library.
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Let's jump back into the Python interactive shell::
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>>> from django.models.polls import polls, choices
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# Make sure our __repr__() addition worked.
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>>> polls.get_list()
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[What's up?]
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# Django provides a rich database lookup API that's entirely driven by
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# keyword arguments.
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>>> polls.get_object(id__exact=1)
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What's up
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>>> polls.get_object(question__startswith='What')
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What's up
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>>> polls.get_object(pub_date__year=2005)
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What's up
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>>> polls.get_object(id__exact=2)
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Traceback (most recent call last):
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...
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django.models.polls.PollDoesNotExist: Poll does not exist for {'id__exact': 2}
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>>> polls.get_list(question__startswith='What')
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[What's up]
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# Give the Poll a couple of Choices. Each one of these method calls does an
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# INSERT statement behind the scenes and returns the new Choice object.
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>>> p = polls.get_object(id__exact=1)
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>>> p.add_choice(choice='Not much', votes=0)
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Not much
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>>> p.add_choice(choice='The sky', votes=0)
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The sky
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>>> c = p.add_choice(choice='Just hacking again', votes=0)
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# Choice objects have API access to their related Poll objects.
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>>> c.get_poll()
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What's up
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# And vice versa: Poll objects get access to Choice objects.
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>>> p.get_choice_list()
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[Not much, The sky, Just hacking again]
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>>> p.get_choice_count()
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3
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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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>>> choices.get_list(poll__pub_date__year=2005)
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[Not much, The sky, Just hacking again]
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# Let's delete one of the choices. Use delete() for that.
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>>> c = p.get_choice(choice__startswith='Just hacking')
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>>> c.delete()
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For full details on the database API, see our `Database API reference`_.
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.. _Database API reference: http://www.djangoproject.com/documentation/db_api/
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Coming soon
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===========
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The tutorial ends here for the time being. But check back within 48 hours for
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more:
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* Using the dynamically-generated admin site
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* Writing public-facing apps
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* Using the cache framework
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* Using the RSS framework
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