Django app that provides an easy way of extending or overriding behaviour of ModelAdmin classes that have already been registered by other apps
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Django app that provides an easy way of extending or overriding behaviour of ModelAdmin classes that have already been registered by other apps.

This is usually useful when the ModelAdmin you're altering is part of a third-party app that you can't/wouldn't want to fork.

Extending model admins

Let's assume we have a django project which uses django.contrib.auth.

Most of the times you have a many-to-many relation you would like your selection field to use filter_horizontal. However, django.contrib.auth.admin.UserAdmin uses the simple <select multiple> which is incredibly annoying to use.

To be able to use filter_horizontal with UserAdmin you can use django-admin-extend in the admin module of one of the apps you write:

from django.contrib.auth.models import User

class ExtendedUserAdmin(registered_modeladmin(User)):
    filter_horizontal = ('user_permissions', 'groups')

You can also override a ModelAdmin's ModelForm:

from django.core.exceptions import ValidationError

class ExtendedUserForm(registered_form(User)):

    def clean_username(self):
        username = self.cleaned_data.get('username', None)
        if username and not username.isalpha():
            raise ValidationError('Invalid username. Only alphabetic characters allowed')
        return username

The advantage of using registered_modeladmin over explicitly inheriting from the ModelAdmin is the fact that multiple apps can override functionality and remain decoupled.

Note: The order of in the INSTALLED_APPS setting matters. The app that uses extend_registered needs to be after the app that first defines and registers the ModelAdmin.

The alternative would be to use explicit inheritance:

# app1.admin
class App1UserAdmin(UserAdmin):
# app2.admin
class App2UserAdmin(App1UserAdmin):

But this creates a dependency between app2 and app1.

Bidirectional many to many fields

This is a generic mechanism for implementing an old feature request that never got into django.

Let's assume you're using django.contrib.sites and one of your models (let's say Snippet) has a many to many relation with the Site model. Whenever you add a new site, you want to be able to assign snippets to that new site.

You can do this by using add_bidirectional_m2m:

class Snippet(models.Model):
    name = models.CharField(unique=True, max_length=255)
    sites = models.ManyToManyField(Site, null=False, blank=True)
class ExtendedSiteAdminForm(add_bidirectional_m2m(registered_form(Site))):

    snippets = ModelMultipleChoiceField(

    def _get_bidirectional_m2m_fields(self):
        return super(ExtendedSiteAdminForm, self).\
            _get_bidirectional_m2m_fields() + [('snippets', 'smartsnippet_set')]

_get_bidirectinal_m2m_fields needs to return a list of tuples, where each tuple contains the form field name and the related manager's name.

Mind the fact that if the object you're saving is new, then the form's save method will cause a database save regardless of the value of the commit parameter.

The extension above can also be made from multiple apps, each injecting their own bidirectional many to many fields.

See bidirectional_many_to_many.png for an example of how this would look.