Question

I have this structure of model objects:

Class A:

b = models.ManyToManyField("B")

Class B:

c = models.ForeignKey("C")
d = models.ForeignKey("D")

Class C:

d = models.ForeignKey("D")

This is the query I’m trying to get:
I want to get all the B objects of object A, then in each B object to perform a comparison between the D object and the c.d object.

I know that simply move on the B collection with for loop and make this comparison.
But I dived on the ManyToMany relation, then I noticed I can do the following:

bObjects = A.objects.all().b

q = bObjects.filter(c__d=None)

This is working, it gives me all the c objects with None d field. But when I try the following :

q = bObjects.filter(c__d=d)

It gives me d not defined, but d is an object like c in the object B.

What can be the problem? I’ll be happy if you suggest further way to do this task.
I generally I’m trying to write my query in a single operation with many to many sub objects and not using loops.

Answer 1

q = bObjects.filter(c_d=d) //Give me d not defined. but d is an object like c in the object B.

Try this:

from django.db.models import F
q = bObjects.filter(c__d=F('d'))

As for the question from your comment below you can have 1 sql query instead of 100 in those ways:

1) if you can express your selection of A objects in terms of a query (for example a.price<10 and a.weight>20) use this:

B.objects.filter(a__price__lt=10, a__weight__gt=20, c__d=F('d'))

or this:

B.objects.filter(a__in=A.objects.filter(price__lt=10, weight__gt=20), c_d=F('d'))

2) if you just have a python list of A objects, use this:

B.objects.filter(a__pk__in=[a.pk for a in your_a_list], c__d=F('d'))