django, manytomany through table with genric foreignkey? - django

Question

Suppose ModelA and (ModelB1, ModelB2 but they don't have common ancestor) has manytomany relationships.
Now I have a through model ModelAtoB.
class ModelAToB(Model):
model_a = models.ForeignKey(ModelA)
content_type=models.ForeignKey(ContentType)
object_id = models.PositiveIntegerField()
model_b = generic.GenericForeignKey('content_type', 'object_id')
I see ManyToManyField being used in the django doc, how can I use it in my case as well?
class ModelA(Model):
pass
class ModelB(Model): # hum.. there's no ModelB which is a common ancestor of ModelB1, ModelB2
model_a_list = models.ManyToManyField(ModelA, through='ModelAtoB')

Answer 1

You cannot use the ManyToManyField in your case however your through model (ModelAToB) is enough to store the relation.
If you want to query all related ModelBs models to a given ModelAs you would do it somehow like this:
related_b = ModelAToB.objects.filter(model_a=a).prefetch_related('model_b')
b_instances = [rel.model_b for rel in related_b]
If you want all related ModelAs to a given ModelB this would be done this way:
from django.contrib.contenttypes.models import ContentType
model_b_ct = ContentType.objects.get_for_model(b)
related_a = ModelAToB.objects.filter(object_id=b.id, content_type=model_b_ct)
a_instance = [rel.model_a for rel in related_a]
Making a model method might be a handy tweak.
class ModelB(Model)::
def model_a_list(self):
return ModelA.objects.filter(
modelatob__content_type__app_label=self._meta.app_label,
modelatob__content_type__model=self._meta.module_name,
modelatob__object_id=self.id
)
This returns a queryset so you still can do things like .count(), .filter() on it.