::-- limodou [2005-07-20 01:52:39]

1. Writing your first Django app, part 2

By Adrian Holovaty <[email protected]>

This tutorial begins where Tutorial 1 left off. We're continuing the Web-poll application and will focus on Django's automatically-generated admin site.

1.1. Philosophy

Generating admin sites for your staff or clients to add, change and delete content is tedious work that doesn't require much creativity. For that reason, Django entirely automates creation of admin interfaces for models.

Django was written in a newsroom environment, with a very clear separation between "content publishers" and the "public" site. Site managers use the system to add news stories, events, sports scores, etc., and that content is displayed on the public site. Django solves the problem of creating a unified interface for site administrators to edit content.

The admin isn't necessarily intended to be used by site visitors; it's for site managers.

1.2. Start the development server

To make things easy, Django comes with a pure-Python Web server that builds on the BaseHTTPServer included in Python's standard library. Let's start the server and explore the admin site. First, set the DJANGO_SETTINGS_MODULE environment variable to the location of your admin settings:

export DJANGO_SETTINGS_MODULE=myproject.settings.admin

Then, run this command to start the server:

django-admin.py runserver

It'll start a Web server running locally -- on port 8000, by default. If you want to change the server's port, pass it as a command-line argument:

django-admin.py runserver 8080

DON'T use this server in anything resembling a production environment. It's intended only for use while developing.

Now, open a Web browser and go to "/admin/" on your domain. You should see the admin's login screen:admin01.png

1.3. Create a user account

You can't log in, though, because you haven't created an admin user account yet. Drop into the Python interactive interpreter and type this:

# The function django.models.auth.users.create_user() creates a new user
# and returns the new auth.User object.
# Don't use 'username' and 'password'. Those are just examples.
>>> from django.models.auth import users
>>> u = users.create_user('username', '[email protected]', 'password')

# But we're not done. We need to explicitly set is_staff and is_active to
# allow this user to access the admin. Might as well make it a superuser,
# too.
u.is_staff = True
u.is_active = True
u.is_superuser = True

# Remember, call the save() method to save changes.
u.save()

1.4. Enter the admin site

Now, try logging in.

If it didn't work, read the "I can't log in" questions in the FAQ.

If it worked, you should see the Django admin index page:admin02t.png

By default, you should see four types of editable content: groups, users, redirects and flat files. These are core features Django ships with by default.

1.5. Make the poll app modifiable in the admin

But where's our poll app? It's not displayed on the admin index page.

Just one thing to do: We need to specify in the polls.Poll model that Poll objects have an admin interface. Edit the myproject/apps/polls/models/polls.py file and make the following change to add an admin attribute:

class Poll(meta.Model):
    fields = (
        # ...
    )
    admin = meta.Admin(
        fields = (
            (None, {'fields': ('question', 'pub_date')}),
        ),
    )

Restart your development Web server, and reload the Django admin page. You'll have to restart the server each time you make a change to Python code -- but we're working on changing that.

1.6. Explore the free admin functionality

Now that Poll has the admin attribute, Django knows that it should be displayed on the admin index page:admin03t.png

Click "Polls." Now you're at the "change list" page for polls. This page displays all the polls in the database and lets you choose one to change it. There's the "What's up?" poll we created in the first tutorial:admin04t.png

Click the "What's up?" poll to edit it:admin05t.png

Things to note here:

• The form is automatically generated from the Poll model.

• The different model field types (meta.DateTimeField, meta.CharField) correspond to the appropriate HTML input widget. Each type of field knows how to display itself in the Django admin.

• Each DateTimeField gets free JavaScript shortcuts. Dates get a "Today" shortcut and calendar popup, and times get a "Now" shortcut and a convenient popup that lists commonly entered times.

The bottom part of the page gives you a couple of options:

• Save -- Saves changes and returns to the change-list page for this type of object.
• Save and continue editing -- Saves changes and reloads the admin page for this object.
• Save and add another -- Saves changes and loads a new, blank form for this type of object.
• Delete -- Displays a delete confirmation page.

Change the "Date published" by clicking the "Today" and "Now" shortcuts. Then click "Save and continue editing." Then click "History" in the upper right. You'll see a page listing all changes made to this object via the Django admin, with the timestamp and username of the person who made the change:admin06t.png

1.7. Customize the admin form

Take a few minutes to marvel at all the code you didn't have to write.

Let's customize this a bit. We can reorder the fields by changing the order of the field names in the admin attribute of the model:

admin = meta.Admin(
    fields = (
        (None, {'fields': ('pub_date', 'question')}),
    ),
)

That made the "Publication date" show up first instead of second:admin07.png

This isn't impressive with only two fields, but for admin forms with dozens of fields, choosing an intuitive order is an important usability detail.

And speaking of forms with dozens of fields, you might want to split the form up into fieldsets:

admin = meta.Admin(
    fields = (
        (None, {'fields': ('question',)}),
        ('Date information', {'fields': ('pub_date',)}),
    ),
)

The first element of each tuple in fields is the title of the fieldset. Here's what our form looks like now:admin08t.png

You can assign arbitrary HTML classes to each fieldset. Django provides a "collapse" class that displays a particular fieldset initially collapsed. This is useful when you have a long form that contains a number of fields that aren't commonly used:

admin = meta.Admin(
    fields = (
        (None, {'fields': ('question',)}),
        ('Date information', {'fields': ('pub_date',), 'classes': 'collapse'}),
    ),
)

admin09.png

1.8. Adding related objects

OK, we have our Poll admin page. But a Poll has multiple Choices, and the admin page doesn't display choices.

Yet.

In this case, there are two ways to solve this problem. The first is to give the Choice model its own admin attribute, just as we did with Poll. Here's what that would look like:

class Choice(meta.Model):
    # ...
    admin = meta.Admin(
        fields = (
            (None, {'fields': ('poll_id', 'choice', 'votes')}),
        ),
    )

(Note that we used "poll_id" to refer to the ForeignKey(Poll) field. The field name is automatically calculated from the model's class name, lowercased, plus '_id'.)

Now "Choices" is an available option in the Django admin. The "Add choice" form looks like this:admin10.png

In that form, the "Poll" field is a select box containing every poll in the database. In our case, only one poll exists at this point.

Also note the "Add Another" link next to "Poll." Every object with a ForeignKey relationship to another gets this for free. When you click "Add Another," you'll get a popup window with the "Add poll" form. If you add a poll in that window and click "Save," Django will save the poll to the database and dynamically add it as the selected choice on the "Add choice" form you're looking at.

But, really, this is an inefficient way of adding Choice objects to the system. It'd be better if you could add a bunch of Choices directly when you create the Poll object. Let's make that happen.

Remove the admin for the Choice model. Then, edit the ForeignKey(Poll) field like so:

meta.ForeignKey(Poll, edit_inline=True, num_in_admin=3),

This tells Django: "Choice objects are edited on the Poll admin page. By default, provide enough fields for 3 Choices."

Then change the other fields in Choice to give them core=True:

meta.CharField('choice', 'choice', maxlength=200, core=True),
meta.IntegerField('votes', 'votes', core=True),

This tells Django: "When you edit a Choice on the Poll admin page, the 'choice' and 'votes' fields are required. The presence of at least one of them signifies the addition of a new Choice object, and clearing at least one of them signifies the deletion of that existing Choice object."

Load the "Add poll" page to see how that looks:admin11t.png

It works like this: There are three slots for related Choices -- as specified by num_in_admin -- but each time you come back to the "Change" page for an already-created object, you get one extra slot. (This means there's no hard-coded limit on how many related objects can be added.) If you wanted space for three extra Choices each time you changed the poll, you'd use num_extra_on_change=3.

One small problem, though. It takes a lot of screen space to display all the fields for entering related Choice objects. For that reason, Django offers an alternate way of displaying inline related objects:

meta.ForeignKey(Poll, edit_inline=True, num_in_admin=3, edit_inline_type=meta.TABULAR),

With that edit_inline_type=meta.TABULAR, the related objects are displayed in a more compact, table-based format:admin12.png

1.9. Customize the admin change list

Now that the Poll admin page is looking good, let's make some tweaks to the "change list" page -- the one that displays all the polls in the system.

Here's what it looks like at this point:admin04t.png

By default, Django displays the repr() of each object. But it'd be more helpful if we could display individual fields. To do that, use the list_display option, which is a tuple of field names to display, as columns, on the change list page for the object:

class Poll(meta.Model):
    # ...
    admin = meta.Admin(
        fields = (
            (None, {'fields': ('question', 'pub_date')}),
        ),
        list_display = ('question', 'pub_date'),
    )

Just for good measure, let's also include the was_published_today custom method from Tutorial 1:

list_display = ('question', 'pub_date', 'was_published_today'),

Now the poll change list page looks like this:admin13t.png

You can click on the column headers to sort by those values -- except in the case of the was_published_today header, because sorting by the output of an arbitrary method is not supported. Also note that the column header for was_published_today is, by default, the name of the method. But you can change that by giving that method a short_description attribute:

def was_published_today(self):
    return self.pub_date.date() == datetime.date.today()
was_published_today.short_description = 'Was published today'

Let's add another improvement to the Poll change list page: Filters. Add the following line to Poll.admin:

list_filter = ('pub_date', )

That adds a "Filter" sidebar that lets people filter the change list by the pub_date field:admin14t.png

The type of filter displayed depends on the type of field you're filtering on. Because pub_date is a DateTimeField, Django knows to give the default filter options for DateTimeFields: "Any date," "Today," "Past 7 days," "This month," "This year." Explore using list_filter on other types of fields.

This is shaping up well. Let's add some search capability:

search_fields = ('question', )

That adds a search box at the top of the change list. When somebody enters search terms, Django will search the question field. You can use as many fields as you'd like -- although because it uses a LIKE query behind the scenes, keep it reasonable, to keep your database happy.

Finally, because Poll objects have dates, it'd be convenient to be able to drill down by date. Add this line:

date_hierarchy = 'pub_date',

That adds hierarchical navigation, by date, to the top of the change list page. At top level, it displays all available years. Then it drills down to months and, ultimately, days.

Now's also a good time to note that change lists give you free pagination. The default is to display 50 items per page. Change-list pagination, search boxes, filters, date-hierarchies and column-header-ordering all work together like you think they should.

1.10. Customize the admin look and feel

Clearly having "Django administration" and "example.com" at the top of each admin page is ridiculous. It's just placeholder text.

That's easy to change, though, using Django's template system.

Open your admin settings file and look at the TEMPLATE_DIRS setting. TEMPLATE_DIRS is a tuple of filesystem directories to check when loading Django templates. It's a search path.

The django-admin.py startproject command automatically prepopulated this setting with the location of Django's default admin templates, according to where you have Django installed. But let's add an extra line to TEMPLATE_DIRS so that it checks a custom directory first, before checking the default admin template directory:

TEMPLATE_DIRS = (
    "/home/mytemplates/admin",
    "/usr/lib/python2.3/site-packages/django/conf/admin_templates",
)

Now copy the template base_site.html from within the default Django admin template directory, into /home/mytemplates/admin (or wherever you're putting your custom admin templates). Edit the file and replace the generic Django stuff with your own site's name as you see fit.

Note that any of Django's default admin templates can be overridden. To override a template, just do the same thing you did with base_site.html -- copy it from the default directory into your custom directory, and make changes.

1.11. Customize the admin index page

On a similar note, you might want to customize the look and feel of the Django admin index page.

By default, it displays all available apps, according to your INSTALLED_APPS setting. But the order in which it displays things is random, and you may want to make significant changes to the layout. After all, the index is probably the most important page of the admin, and it should be easy to use.

The template to customize is index.html. (Do the same as with base_site.html in the previous section -- copy it from the default directory to your custom template directory.) Edit the file, and you'll see it uses a template tag called {% get_admin_app_list as app_list %}. That's the magic that retrieves every installed Django app. Instead of using that, you can hard-code links to object-specific admin pages in whatever way you think is best.

Django offers another shortcut in this department. Run the command django-admin.py adminindex polls to get a chunk of template code for inclusion in the admin index template. It's a useful starting point.

For full details on customizing the look and feel of the Django admin site in general, see the Django admin CSS guide.

1.12. Coming soon

The tutorial ends here for the time being. But check back within 48 hours for the next installments:

• Writing public-facing apps
• Using the cache framework
• Using the RSS framework
• Using the comments framework