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tutorial01.txt
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=====================================
Writing your first Django app, part 1
=====================================
By Adrian Holovaty <holovaty@gmail.com>
Let's learn by example.
Throughout this tutorial, we'll walk you through the creation of a simple Web
poll application.
It'll consist of two parts:
* A public site that lets people vote in polls and view poll results.
* An admin site that lets you add, change and delete polls behind the scenes.
We'll assume you have `Django installed`_ already.
.. _`Django installed`: http://www.djangoproject.com/documentation/install/
Initial setup
=============
If this is your first time using Django, you'll have to take care of some
initial setup.
Run the command ``django-admin.py startproject myproject``. That'll create a
``myproject`` directory in your current directory.
(``django-admin.py`` should be on your path if you installed Django via
its setup.py utility. If it's not on your path, you can find it in
``site-packages/django/bin``; consider symlinking to it from some place
on your path, such as /usr/local/bin.)
A project is a collection of settings for an instance of Django -- including
database configuration, Django-specific options and application-specific
settings. Let's look at what ``startproject`` created::
$ cd myproject/
$ ls
apps/ __init__.py settings/
$ ls settings/
__init__.py admin.py main.py
# ls apps/
__init__.py
First, edit ``myproject/settings/main.py``. It's a normal Python module with
module-level variables representing Django settings. Edit the file and change
these settings to match your database's connection parameters:
* ``DATABASE_ENGINE`` -- Either 'postgresql' or 'mysql'. More coming soon.
* ``DATABASE_NAME`` -- The name of your database.
* ``DATABASE_USER`` -- Your database username.
* ``DATABASE_PASSWORD`` -- Your database password.
* ``DATABASE_HOST`` -- The host your database is on. Leave this as an
empty string if your database server is on the same physical machine
(localhost).
(Make sure you've created a database within PostgreSQL or MySQL by this point.
Do that with "``CREATE DATABASE database_name;``" within your database's
interactive prompt.)
Also, note that MySQL support is a recent development, and Django hasn't been
comprehensively tested with that database. If you find any bugs in Django's
MySQL bindings, please file them in `Django's ticket system`_ so we can fix them
immediately.
Once you've done that, you need to tell Django which settings module you're
currently using. Do that by setting an environment variable,
``DJANGO_SETTINGS_MODULE``. Here's how you do that in the Bash shell on Unix::
export DJANGO_SETTINGS_MODULE=myproject.settings.main
On Windows, you'd use ``set`` instead::
set DJANGO_SETTINGS_MODULE=myproject.settings.main
Note this path is in Python package syntax. Your project has to be somewhere on
your `Python path`_ -- so that the Python statement ``import myproject.settings.main``
works. Throughout Django, you'll be referring to your projects and apps via
Python package syntax.
Then run the following command::
django-admin.py init
If you don't see any errors, you know it worked. That command initialized your
database with Django's core database tables. If you're interested, run the
PostgreSQL or MySQL command-line client and type "\\dt" (PostgreSQL) or
"SHOW TABLES;" (MySQL) to display the tables.
Now you're set to start doing work. You won't have to take care of this boring
administrative stuff again.
.. _`Python path`: http://docs.python.org/tut/node8.html#SECTION008110000000000000000
.. _`Django's ticket system: http://code.djangoproject.com/report/1
Creating models
===============
Change into the ``myproject/apps`` directory and type this command::
django-admin.py startapp polls
That'll create a directory structure like this::
polls/
__init__.py
models/
__init__.py
polls.py
urls/
__init__.py
polls.py
views/
__init__.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.
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.
Edit the ``polls/models/polls.py`` file so that it looks like this::
from django.core import meta
class Poll(meta.Model):
fields = (
meta.CharField('question', 'question', maxlength=200),
meta.DateTimeField('pub_date', 'date published'),
)
class Choice(meta.Model):
fields = (
meta.ForeignKey(Poll),
meta.CharField('choice', 'choice', maxlength=200),
meta.IntegerField('votes', 'votes'),
)
The code is straightforward. Each model is represented by a class that
subclasses ``django.core.meta.Model``. Each model has a single class variable,
``fields``, which is a tuple of database fields in the model.
Each field is represented by an instance of a ``meta.*Field`` class -- e.g.,
``meta.CharField`` for character fields and ``meta.DateTimeField`` for
datetimes. This tells Django what type of data each field holds.
The first argument to each ``Field`` call 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.
The second argument is the field's human-readable name. That's used in a couple
of introspective parts of Django, and it doubles as documentation.
Some ``meta.*Field`` classes have additional required elements.
``meta.CharField``, for example, requires that you give it a ``maxlength``.
That's used not only in the database schema, but in validation, as we'll soon
see.
Finally, note a relationship is defined, using ``meta.ForeignKey``. That tells
Django each Choice is related to a single Poll. Django supports all the common
database relationships: many-to-ones, many-to-manys and one-to-ones.
.. _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.
PHILOSOPHY: Django apps are "pluggable": You can use an app in multiple
projects, and you can distribute apps, because they're not tied to a given
Django installation.
Edit the myproject/settings/main.py file again, and change the ``INSTALLED_APPS``
setting to include the string "myproject.apps.polls". So it'll look like this::
INSTALLED_APPS = (
'myproject.apps.polls',
)
(Don't forget the trailing comma because of Python's rules about single-value
tuples.)
Now Django knows myproject includes the polls app. Let's run another command::
django-admin.py sql polls
You should see the following (the CREATE TABLE SQL statements for the polls app)::
BEGIN;
CREATE TABLE polls_polls (
id serial NOT NULL PRIMARY KEY,
question varchar(200) NOT NULL,
pub_date timestamp with time zone NOT NULL
);
CREATE TABLE polls_choices (
id serial NOT NULL PRIMARY KEY,
poll_id integer NOT NULL REFERENCES polls_polls (id),
choice varchar(200) NOT NULL,
votes integer NOT NULL
);
COMMIT;
Note the following:
* Table names are automatically generated by combining the name of the app
(polls) with a plural version of the object name (polls and choices). (You
can override this behavior.)
* Primary keys (IDs) are added automatically. (You can override this, too.)
* 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) vs. ``serial`` (PostgreSQL) are handled
for you automatically. The author of this tutorial runs PostgreSQL, so the
example output is in PostgreSQL syntax.
If you're interested, also run the following commands:
* ``django-admin.py sqlinitialdata polls`` -- Outputs the initial-data inserts
required for Django's admin framework.
* ``django-admin.py sqlclear polls`` -- Outputs the ``DROP TABLE`` statements
for this app.
* ``django-admin.py sqlindexes polls`` -- Outputs the ``CREATE INDEX``
statements for this app.
* ``django-admin.py sqlall polls`` -- A combination of 'sql' and
'sqlinitialdata'.
Looking at the output of those commands can help you understand what's actually
happening under the hood.
Now, run this command::
django-admin.py install polls
That command automatically creates the database tables for the polls app.
Behind the scenes, all it does is take the output of
``django-admin.py sqlall polls`` and execute it in the database pointed-to by
your Django settings file.
Playing with the API
====================
Now open the Python interactive shell, and play around with the free Python API
Django gives you::
# Modules are dynamically created within django.models.
# Their names are plural versions of the model class names.
>>> from django.models.polls import polls, choices
# No polls are in the system yet.
>>> polls.get_list()
[]
# Create a new Poll.
>>> from datetime import datetime
>>> p = polls.Poll(id=None, 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.
>>> p.id
1
# Access database columns via Python attributes.
>>> p.question
"What's up?"
>>> p.pub_date
datetime.datetime(2005, 7, 15, 12, 00, 53)
# Change values by changing the attributes, then calling save().
>>> p.pub_date = datetime(2005, 4, 1, 0, 0)
>>> p.save()
# get_list() displays all the polls in the database.
>>> polls.get_list()
[<Poll object>]
Wait a minute. ``<Poll object>`` is, utterly, an unhelpful representation of
this object. Let's fix that by editing the polls model and adding a
``__repr__()`` method to both ``Poll`` and ``Choice``::
class Poll(meta.Model):
# ...
def __repr__(self):
return self.question
class Choice(meta.Model):
# ...
def __repr__(self):
return self.choice
It's important to add ``__repr__()`` methods to your models, not only for your
own sanity when dealing with the interactive prompt, but also because objects'
representations are used throughout Django's automatically-generated admin.
Note these are normal Python methods. Let's add a custom method, just for
demonstration::
class Poll(meta.Model):
# ...
def was_published_today(self):
return self.pub_date.date() == datetime.date.today()
Note ``import datetime`` wasn't necessary. Each model method has access to
a handful of commonly-used variables for convenience, including the
``datetime`` module from the Python standard library.
Let's jump back into the Python interactive shell::
>>> from django.models.polls import polls, choices
# Make sure our __repr__() addition worked.
>>> polls.get_list()
[What's up?]
# Django provides a rich database lookup API that's entirely driven by
# keyword arguments.
>>> polls.get_object(id__exact=1)
What's up
>>> polls.get_object(question__startswith='What')
What's up
>>> polls.get_object(pub_date__year=2005)
What's up
>>> polls.get_object(id__exact=2)
Traceback (most recent call last):
...
PollDoesNotExist: Poll does not exist for {'id__exact': 2}
>>> polls.get_list(question__startswith='What')
[What's up]
# Make sure our custom method worked.
>>> p = polls.get_object(id__exact=1)
>>> p.was_published_today()
False
# Give the Poll a couple of Choices. Each one of these method calls does an
# INSERT statement behind the scenes and returns the new Choice object.
>>> p = polls.get_object(id__exact=1)
>>> p.add_choice(choice='Not much', votes=0)
Not much
>>> p.add_choice(choice='The sky', votes=0)
The sky
>>> c = p.add_choice(choice='Just hacking again', votes=0)
# Choice objects have API access to their related Poll objects.
>>> c.get_poll()
What's up
# And vice versa: Poll objects get access to Choice objects.
>>> p.get_choice_list()
[Not much, The sky, Just hacking again]
>>> p.get_choice_count()
3
# The API automatically follows relationships as far as you need.
# Use double underscores to separate relationships.
# This works as many levels deep as you want. There's no limit.
# Find all Choices for any poll whose pub_date is in 2005.
>>> choices.get_list(poll__pub_date__year=2005)
[Not much, The sky, Just hacking again]
# Let's delete one of the choices. Use delete() for that.
>>> c = p.get_choice(choice__startswith='Just hacking')
>>> c.delete()
For full details on the database API, see our `Database API reference`_.
When you're comfortable with the API, read `part 2 of this tutorial`_ to get
Django's automatic admin working.
.. _Database API reference: http://www.djangoproject.com/documentation/db_api/
.. _part 2 of this tutorial: http://www.djangoproject.com/documentation/tutorial2/