Using another model manager's code in a Django custom manager class - django

Question

I have two models, say, Question and Topic.
I am trying to add methods to the Question model's custom manager, e.g. some method that filters by Topic.
I can't seem to use the other manager's code for this (cannot import Topic either, so I can't do Topic.objects...)
In class QuestionManager
def my_feed(self, user):
topics = TopicManager().filter(user=user) # 1st approach
#topics = Topic.objects.filter(user=user) # 2nd line
# do something with topics
class TopicManager
....
Using 1st approach, I get the following error:
virtualenv/local/lib/python2.7/site-packages/django/db/models/sql/query.pyc in get_meta(self)
219 by subclasses.
220 """
--> 221 return self.model._meta
222
223 def clone(self, klass=None, memo=None, **kwargs):
AttributeError: 'NoneType' object has no attribute '_meta'
I can't use the 2nd line since I can't import Topic, since Topic depends on the TopicManager in this file. Is there a workaround for this?

Answer 1

You can't use a manager directly, in any circumstance. You always access it via the model class.
If you can't import the model at the top of the file because of a circular dependency, you can simply import it inside the method.

Answer 2

You should be able to place this at the bottom of the managers.py module:
# Prevent circular import error between models.py and managers.py
from apps.drs import models
In your manager classes you can reference other models using models.<modelname>, and this should work, avoiding the circular import.
For example:
class QuestionManager(Manager):
def my_feed(self, user):
topics = models.Topic.objects.filter(user=user)
# do something with topics
# Prevent circular import error between models.py and managers.py
from apps.drs import models
This works because you are importing the module, not the model classes, which results in a lazy import. By the time the function is run the module will be imported and everything will work.
You could also load models by name using django.apps.get_model():
from django.apps import apps
apps.get_model('my_app', 'MyModel')
Details here
For example:
from django.apps import apps
class QuestionManager(Manager):
def my_feed(self, user):
topics = apps.get_model('my_app', 'Topic').objects.filter(user=user)
# do something with topics