Importing a model from one app into another app causes "AlreadyRegistered" exception.

[Manuel <manny@…>]

example:

in a project with two apps, app1 and app2, with the following models.py files:

app1/models.py:

from django.db import models
from django.contrib import admin

class MyFirstModel(models.Model):
    pass

admin.site.register(MyFirstModel)

...

app2/models.py

from django.db import models
from django.contrib import admin
from app1.models import MyFirstModel

...

running the command:

./manage.py runserver

raises the following exception:

Unhandled exception in thread started by <function inner_run at 0x2ab13a637c80>
Traceback (most recent call last):
...
 File "/usr/local/lib/python2.4/site-packages/django/contrib/admin/sites.py", line 82, in register
    raise AlreadyRegistered('The model %s is already registered' % model.__name__)
django.contrib.admin.sites.AlreadyRegistered: The model MyFirstModel is already registered

My quick and dirty solution is to get rid of the exception in the register method of the AdminSite class in the sites.py file. i.e. changing it from this:

for model in model_or_iterable:
     if model in self._registry:
        raise AlreadyRegistered('The model %s is already registered' % model.__name__)
     self._registry[model] = admin_class(model, self)

to this:

for model in model_or_iterable:
    if not model in self._registry:
        self._registry[model] = admin_class(model, self)

In this way models are only registered if they haven't already been registered, and are ignored otherwise (instead of raising an exception) - as far as I can tell this shouldn't be a problem - but I'm not familiar enough with the code and your objectives to know for sure.

[Brian Rosner]

I am still not 100% sure if this is a problem. Let me first explain why this is occuring. You are performing an import from app1.models import MyFirstModel and you probably have this app in your INSTALLED_APPS using the project. For example:

INSTALLED_APPS = (
    "project.app1",
)

This will result in Python importing the models twice. One in each import. If you change the first import to use the project it will work or vise-versa.

[mrts]

I was bitten by this as well by using relative imports (from app import models etc). Once I replaced relative imports with absolute imports (from project.app import models) everything works well.

This error is actually good for detecting unnecessary double imports, so I'd be -1 for removing it. Perhaps the error should provide a hint how to solve the problem instead, e.g. "The model MyFirstModel is already registered. This is probably caused by a double import, see docs/foo for ways to avoid double imports" as this is in no way obvious.

[Simon Willison]

Another call for this behaviour to be changed. It's horrible to debug, and there doesn't seem to be any reason not to just ignore attempted dual registrations. Taking this to the mailing list.

[Simon Willison]

See ​http://groups.google.com/group/django-developers/browse_thread/thread/2cc3a9dc358138af

[Simon Willison]

This was discussed a few weeks ago:

​http://groups.google.com/group/django-developers/browse_thread/thread/1a40c64e9c952c48

Consensus seemed to be for option number 2, quoted here:

Throw the exception based on whether the ModelAdmin is different
from a previously registered one. This will technically keep the
current behavior, but is really special casing the instances where
admin.site.register is called more than once. This currently happens
every so often with the models.py which is the documented method of
registration.

Solving this requires being able to compare two ModelAdmin subclasses for equality, but as Malcolm points out this will happen only on startup/import so doesn't need to be very efficient.

[Brian Rosner]

Marking with the correct keyword and stage. I will be working on a fix for this.

[Brian Rosner]

initial cut for patching this. needs alot more work, but this is a proof of concept for the final patch.

[Brian Rosner]

slightly better patch and tests

[Brian Rosner]

It would seem to me that my patch is prone to some threading issues. Anyone want to shed some light on this?

[Brian Rosner]

(In [7872]) newforms-admin: Added autodiscover functionality to django.contrib.admin. This makes the admin aware of per-app admin.py modules and does an import on them when explicitly called. Docs show how this is used. Fixed #6003, #6776, #6776.

[Jacob]

Milestone 1.0 alpha deleted

[Sacha Zyto <sachazyto@…>]

Replying to mrts:

Once I replaced relative imports with absolute imports (from project.app import models) everything works well.

Note that if you prefer, you can use only relative imports and it will work as well (just tested it on django1.3.1). The exception happens when you combine absolute and relative imports.