Model Customization

So far under Content Architecture the concept of subclassing Mezzanine’s models has been described. This section describes the hooks Mezzanine provides for directly modifying the behaviour of its models.

Field Injection

Mezzanine provides the setting EXTRA_MODEL_FIELDS which allows you to define a sequence of fields that will be injected into Mezzanine’s (or any library’s) models.

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Using the following approach comes with certain trade-offs described below in Field Injection Caveats. Be sure to fully understand these prior to using the EXTRA_MODEL_FIELDS setting.

Each item in the EXTRA_MODEL_FIELDS sequence is a four item sequence. The first two items are the dotted path to the model and its field name to be added, and the dotted path to the field class to use for the field. The third and fourth items are a sequence of positional args and a dictionary of keyword args, to use when creating the field instance.

For example suppose you want to inject a custom ImageField from a third party library into Mezzanine’s BlogPost model, you would define the following in your projects’s settings module:

EXTRA_MODEL_FIELDS = (
    # Four-item sequence for one field injected.
    (
        # Dotted path to field.
        "mezzanine.blog.models.BlogPost.image",
        # Dotted path to field class.
        "somelib.fields.ImageField",
        # Positional args for field class.
        ("Image",),
        # Keyword args for field class.
        {"blank": True, "upload_to: "blog"},
    ),
)

Each BlogPost instance will now have an image attribute, using the ImageField class defined in the fictitious somelib.fields module.

Another interesting example would be adding a field to all of Mezzanine’s content types by injecting fields into the Page class. Continuing on from the previous example, suppose you wanted to add a regular Django IntegerField to all content types:

EXTRA_MODEL_FIELDS = (
    (
        "mezzanine.blog.models.BlogPost.image",
        "somelib.fields.ImageField",
        ("Image",),
        {"blank": True, "upload_to": "blog"},
    ),
    # Example of adding a field to *all* of Mezzanine's content types:
    (
        "mezzanine.pages.models.Page.another_field",
        "IntegerField", # 'django.db.models.' is implied if path is omitted.
        ("Another name",),
        {"blank": True, "default": 1},
    ),
)

Note here that the full path for the field class isn’t required since a regular Django field is used - the django.db.models. path is implied.

Field Injection Caveats

The above technique provides a great way of avoiding the performance penalties of SQL JOINS required by the traditional approach of subclassing models, however some extra consideration is required when used with migration tools like South. In the first example above, South views the new image field on the BlogPost model of the mezzanine.blog app. As such in order to create a migration for it, the migration must be created for the blog app itself and by default would end up in the migrations directory of the blog app, which completely goes against the notion of not modifying the blog app to add your own custom fields.

One approach to address this is to use the --stdout argument of South’s schemamigration command, and create your own migration file located somewhere in your project or app:

$ python manage.py schemamigration blog --auto --stdout >> myapp/migrations/0001_blog_customization.py
$ python manage.py migrate myapp

Be warned that over time this approach will almost certainly require some manual intervention by way of editing migrations, or modifying the database manually to create the correct state. Ultimately there is a trade-off involved here.

Admin Fields

Whether using the above approach to inject fields onto models, or taking the more traditional approach of subclassing models, most often you will also want to expose new fields to the admin interface. This can be achieve by simply unregistering the relevant admin class, subclassing it, and re-registering your new admin class for the associated model. Continuing on from the first example, the code below takes a copy of the fieldsets definition for the original BlogPostAdmin, and injects our custom field’s name into the desired position.:

# In myapp/admin.py

from copy import deepcopy
from django.contrib import admin
from mezzanine.blog.admin import BlogPostAdmin
from mezzanine.blog.models import BlogPost

blog_fieldsets = deepcopy(BlogPostAdmin.fieldsets)
blog_fieldsets[0][1]["fields"].insert(-2, "image")

class MyBlogPostAdmin(BlogPostAdmin):
    fieldsets = blog_fieldsets

admin.site.unregister(BlogPost)
admin.site.register(BlogPost, MyBlogPostAdmin)