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::-- limodou [2005-07-17 12:08:15]

Contents

  1. Writing your first Django app, part 1
    1. Initial setup
    2. Creating models
    3. Activating models
    4. Playing with the API

1. Writing your first Django app, part 1

By Adrian Holovaty <[email protected]>

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.

1.1. 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.)

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: 

export 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.

1.2. 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.

1.3. 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.

1.4. 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.