root/django/branches/schema-evolution-ng/docs/overview.txt

==================
Django at a glance
==================

Because Django was developed in a fast-paced newsroom environment, it was
designed to make common Web-development tasks fast and easy. Here's an informal
overview of how to write a database-driven Web app with Django.

The goal of this document is to give you enough technical specifics to
understand how Django works, but this isn't intended to be a tutorial or
reference. Please see our more-detailed Django documentation_ when you're ready
to start a project.

.. _documentation: http://www.djangoproject.com/documentation/

Design your model
=================

Although you can use Django without a database, it comes with an
object-relational mapper in which you describe your database layout in Python
code.

The data-model syntax offers many rich ways of representing your models -- so
far, it's been solving two years' worth of database-schema problems. Here's a
quick example::

    class Reporter(models.Model):
        full_name = models.CharField(maxlength=70)

        def __str__(self):
            return self.full_name

    class Article(models.Model):
        pub_date = models.DateTimeField()
        headline = models.CharField(maxlength=200)
        article = models.TextField()
        reporter = models.ForeignKey(Reporter)

        def __str__(self):
            return self.headline

Install it
==========

Next, run the Django command-line utility to create the database tables
automatically::

    manage.py syncdb

The ``syncdb`` command looks at all your available models and creates tables
in your database for whichever tables don't already exist.

Enjoy the free API
==================

With that, you've got a free, and rich, Python API to access your data. The API
is created on the fly: No code generation necessary::

    >>> from mysite.models import Reporter, Article

    # No reporters are in the system yet.
    >>> Reporter.objects.all()
    []

    # Create a new Reporter.
    >>> r = Reporter(full_name='John Smith')

    # Save the object into the database. You have to call save() explicitly.
    >>> r.save()

    # Now it has an ID.
    >>> r.id
    1

    # Now the new reporter is in the database.
    >>> Reporter.objects.all()
    [John Smith]

    # Fields are represented as attributes on the Python object.
    >>> r.full_name
    'John Smith'

    # Django provides a rich database lookup API.
    >>> Reporter.objects.get(id=1)
    John Smith
    >>> Reporter.objects.get(full_name__startswith='John')
    John Smith
    >>> Reporter.objects.get(full_name__contains='mith')
    John Smith
    >>> Reporter.objects.get(id=2)
    Traceback (most recent call last):
        ...
    DoesNotExist: Reporter does not exist for {'id__exact': 2}

    # Create an article.
    >>> from datetime import datetime
    >>> a = Article(pub_date=datetime.now(), headline='Django is cool',
    ...     article='Yeah.', reporter=r)
    >>> a.save()

    # Now the article is in the database.
    >>> Article.objects.all()
    [Django is cool]

    # Article objects get API access to related Reporter objects.
    >>> r = a.reporter
    >>> r.full_name
    'John Smith'

    # And vice versa: Reporter objects get API access to Article objects.
    >>> r.article_set.all()
    [Django is cool]

    # The API follows relationships as far as you need, performing efficient
    # JOINs for you behind the scenes.
    # This finds all articles by a reporter whose name starts with "John".
    >>> Article.objects.filter(reporter__full_name__startswith="John")
    [Django is cool]

    # Change an object by altering its attributes and calling save().
    >>> r.full_name = 'Billy Goat'
    >>> r.save()

    # Delete an object with delete().
    >>> r.delete()

A dynamic admin interface: It's not just scaffolding -- it's the whole house
============================================================================

Once your models are defined, Django can automatically create a professional,
production ready administrative interface -- a Web site that lets authenticated
users add, change and delete objects. It's as easy as adding a line of code to
your model classes::

    class Article(models.Model):
        pub_date = models.DateTimeField()
        headline = models.CharField(maxlength=200)
        article = models.TextField()
        reporter = models.ForeignKey(Reporter)
        class Admin: pass

The philosophy here is that your site is edited by a staff, or a client, or
maybe just you -- and you don't want to have to deal with creating backend
interfaces just to manage content.

One typical workflow in creating Django apps is to create models and get the
admin sites up and running as fast as possible, so your staff (or clients) can
start populating data. Then, develop the way data is presented to the public.

Design your URLs
================

A clean, elegant URL scheme is an important detail in a high-quality Web
application. Django encourages beautiful URL design and doesn't put any cruft
in URLs, like ``.php`` or ``.asp``.

To design URLs for an app, you create a Python module called a URLconf. A table
of contents for your app, it contains a simple mapping between URL patterns and
Python callback functions. URLconfs also serve to decouple URLs from Python
code.

Here's what a URLconf might look like for the ``Reporter``/``Article``
example above::

    from django.conf.urls.defaults import *

    urlpatterns = patterns('',
        (r'^/articles/(\d{4})/$', 'mysite.views.year_archive'),
        (r'^/articles/(\d{4})/(\d{2})/$', 'mysite.views.month_archive'),
        (r'^/articles/(\d{4})/(\d{2})/(\d+)/$', 'mysite.views.article_detail'),
    )

The code above maps URLs, as simple regular expressions, to the location of
Python callback functions ("views"). The regular expressions use parenthesis to
"capture" values from the URLs. When a user requests a page, Django runs
through each pattern, in order, and stops at the first one that matches the
requested URL. (If none of them matches, Django calls a special-case 404 view.)
This is blazingly fast, because the regular expressions are compiled at load
time.

Once one of the regexes matches, Django imports and calls the given view, which
is a simple Python function. Each view gets passed a request object --
which contains request metadata -- and the values captured in the regex.

For example, if a user requested the URL "/articles/2005/05/39323/", Django
would call the function ``mysite.views.article_detail(request,
'2005', '05', '39323')``.

Write your views
================

Each view is responsible for doing one of two things: Returning an
``HttpResponse`` object containing the content for the requested page, or
raising an exception such as ``Http404``. The rest is up to you.

Generally, a view retrieves data according to the parameters, loads a template
and renders the template with the retrieved data. Here's an example view for
``year_archive`` from above::

    def year_archive(request, year):
        a_list = Article.objects.filter(pub_date__year=year)
        return render_to_response('news/year_archive.html', {'year': year, 'article_list': a_list})

This example uses Django's template system, which has several powerful
features but strives to stay simple enough for non-programmers to use.

Design your templates
=====================

The code above loads the ``news/year_archive.html`` template.

Django has a template search path, which allows you to minimize redundancy among
templates. In your Django settings, you specify a list of directories to check
for templates. If a template doesn't exist in the first directory, it checks the
second, and so on.

Let's say the ``news/article_detail.html`` template was found. Here's what that
might look like::

    {% extends "base.html" %}

    {% block title %}Articles for {{ year }}{% endblock %}

    {% block content %}
    <h1>Articles for {{ year }}</h1>

    {% for article in article_list %}
    <p>{{ article.headline }}</p>
    <p>By {{ article.reporter.full_name }}</p>
    <p>Published {{ article.pub_date|date:"F j, Y" }}</p>
    {% endfor %}
    {% endblock %}

Variables are surrounded by double-curly braces. ``{{ article.headline }}``
means "Output the value of the article's headline attribute." But dots aren't
used only for attribute lookup: They also can do dictionary-key lookup, index
lookup and function calls.

Note ``{{ article.pub_date|date:"F j, Y" }}`` uses a Unix-style "pipe" (the "|"
character). This is called a template filter, and it's a way to filter the value
of a variable. In this case, the date filter formats a Python datetime object in
the given format (as found in PHP's date function; yes, there is one good idea
in PHP).

You can chain together as many filters as you'd like. You can write custom
filters. You can write custom template tags, which run custom Python code behind
the scenes.

Finally, Django uses the concept of "template inheritance": That's what the
``{% extends "base.html" %}`` does. It means "First load the template called
'base', which has defined a bunch of blocks, and fill the blocks with the
following blocks." In short, that lets you dramatically cut down on redundancy
in templates: Each template has to define only what's unique to that template.

Here's what the "base.html" template might look like::

    <html>
    <head>
        <title>{% block title %}{% endblock %}</title>
    </head>
    <body>
        <img src="sitelogo.gif" alt="Logo" />
        {% block content %}{% endblock %}
    </body>
    </html>

Simplistically, it defines the look-and-feel of the site (with the site's logo),
and provides "holes" for child templates to fill. This makes a site redesign as
easy as changing a single file -- the base template.

It also lets you create multiple versions of a site, with different base
templates, while reusing child templates. Django's creators have used this
technique to create strikingly different cell-phone editions of sites -- simply
by creating a new base template.

Note that you don't have to use Django's template system if you prefer another
system. While Django's template system is particularly well-integrated with
Django's model layer, nothing forces you to use it. For that matter, you don't
have to use Django's database API, either. You can use another database
abstraction layer, you can read XML files, you can read files off disk, or
anything you want. Each piece of Django -- models, views, templates -- is
decoupled from the next.

This is just the surface
========================

This has been only a quick overview of Django's functionality. Some more useful
features:

    * A caching framework that integrates with memcached or other backends.
    * A syndication framework that makes creating RSS and Atom feeds as easy as
      writing a small Python class.
    * More sexy automatically-generated admin features -- this overview barely
      scratched the surface.

The next obvious steps are for you to `download Django`_, read `the tutorial`_
and join `the community`_. Thanks for your interest!

.. _download Django: http://www.djangoproject.com/download/
.. _the tutorial: http://www.djangoproject.com/documentation/tutorial1/
.. _the community: http://www.djangoproject.com/community/