How can I add inlines to the ModelAdmin of another app, without a circular dependency?

Question

Let's say I have two models, in different apps. App Two knows about app One, but not the other way around:

# one/models.py

from django.db import models

class One(models.Model):
    pass


# two/models.py

from django.db import models
from one.models import One

class Two(models.Model):
    one = models.ForeignKey(One)

I also have One registered in the admin site:

# one/admin.py

from django.contrib import admin
from .models import One

admin.site.register(One)

How do I register Two as an Inline on One's admin page, without introducing a circular dependency between the two apps?

Answer 1

You can do this pretty simply, providing you don't mind accessing a 'private' attribute on the ModelAdmin. (Attributes beginning with an underscore are treated as private by convention.)

# two/admin.py
from django.contrib import admin
from one.models import One
from .models import Two

class TwoInline(admin.StackedInline):
    model = Two

admin.site._registry[One].inlines.append(TwoInline)