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 system 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:

myproject/
    __init__.py
    apps/
        __init__.py
    settings.py
    urls.py

First, edit myproject/settings.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', 'mysql' or 'sqlite3'. More coming soon.
• DATABASE_NAME -- The name of your database, or the full (absolute) path to the database file if you're using sqlite.
• DATABASE_USER -- Your database username (not used for sqlite).
• DATABASE_PASSWORD -- Your database password (not used for sqlite).
• DATABASE_HOST -- The host your database is on. Leave this as an empty string if your database server is on the same physical machine (not used for sqlite).

Now, take a second to make sure myproject is on your Python path. You can do this by copying myproject to Python's site-packages directory, or you can do it by altering the PYTHONPATH environment variable. See the Python path documentation for more information. If you opt to set the PYTHONPATH environment variable, note that you'll need to set it to the parent directory of myproject. (You can test this by typing "import myproject" into the Python interactive prompt.)

Run the following command:

django-admin.py init --settings=myproject.settings

The django-admin.py utility generally needs to know which settings module you're using. Here, we're doing that by specifying settings= on the command line, but that can get tedious. If you don't want to type settings= each time, you can set the DJANGO_SETTINGS_MODULE environment variable. Here's how you do that in the Bash shell on Unix:

export DJANGO_SETTINGS_MODULE=myproject.settings

On Windows, you'd use set instead:

set DJANGO_SETTINGS_MODULE=myproject.settings

If you don't see any errors after running django-admin.py init, you know it worked. That command initialized your database with Django's core database tables. 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.

Creating models

Now that your environment -- a "project" -- is set up, you're set to start doing work. (You won't have to take care of this boring administrative stuff again.)

Each application you write in Django -- e.g., a weblog system, a database of public records or a simple poll app -- 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.

In this tutorial, we'll create our poll app in the myproject/apps directory, for simplicity. As a consequence, the app will be coupled to the project -- that is, Python code within the poll app will refer to myproject.apps.polls. Later in this tutorial, we'll discuss decoupling your apps for distribution.

To create your app, change into the myproject/apps directory and type this command:

django-admin.py startapp polls

(From now on, this tutorial will leave out the --settings parameter and will assume you've either set your DJANGO_SETTINGS_MODULE environment variable or included the --settings option in your call to the command.)

That'll create a directory structure like this:

polls/
    __init__.py
    models/
        __init__.py
        polls.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.

In our simple poll app, we'll create two models: polls and choices. A poll has a question and a publication date. A choice has two fields: the text of the choice and a vote tally. Each choice is associated with a poll.

These concepts are represented by simple Python classes. Edit the polls/models/polls.py file so it looks like this:

from django.core import meta

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

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

The code is straightforward. Each model is represented by a class that subclasses django.core.meta.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 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 name of each meta.*Field instance (e.g. question or pub_date ) is the field's name, in machine-friendly format. You'll use this value in your Python code, and your database will use it as the column name.

You can use an optional first positional argument to a Field to designate a human-readable name. That's used in a couple of introspective parts of Django, and it doubles as documentation. If this field isn't provided, Django will use the machine-readable name. In this example, we've only defined a human-readable name for Poll.pub_date. For all other fields in this model, the field's machine-readable name will suffice as its human-readable name.

Some meta.*Field classes have 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.

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.

Edit the myproject/settings.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

(Note that it doesn't matter which directory you're in when you run this command.)

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.)
• Django appends "_id" to the foreign key field name, by convention. 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. 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 necessary DROP TABLE statements for this app, according to which tables already exist in your database (if any).
• 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 to create the database tables for the polls app automatically:

django-admin.py install polls

Behind the scenes, all that command does is take the output of django-admin.py sqlall polls and execute it in the database pointed-to by your Django settings file.

Read the django-admin.py documentation for full information on what this utility can do.

Playing with the API

Now, make sure your DJANGO_SETTINGS_MODULE environment variable is set (as explained above), and open the Python interactive shell to 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(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 (in the polls/models/polls.py file) 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?]

# Lookup by a primary key is the most common case, so Django provides a
# shortcut for primary-key exact lookups.
# The following is identical to polls.get_object(id__exact=1).
>>> polls.get_object(pk=1)
What's up?

# Make sure our custom method worked.
>>> p = polls.get_object(pk=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(pk=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.