nested query filter _ Django

Question

I keep it simple. I have 3 models.

 class C(models.model):
    some_field = models.BooleanField(default=False)

 class B(models.model):
    b = models.ForeignKey(C)

 class A(models.model):
    a = models.ForeignKey(B)

I need a query filter that gets A.a.b.some_field = True. how can I achieve this ?

willeM_ Van Onsem · Accepted Answer · 2019-01-26 11:32:40Z

You can filter your A objects that satisfy this condition with:

A.objects.filter(a__b__some_field=True)

This will generate a query that looks, more or less like:

SELECT a.*
FROM a
JOIN b ON a.a_id = b.id
JOIN c ON b.b_id = c.id
WHERE c.some_field = 1

The double underscore (__) can be used to look "through" relations (like ForeignKeys, OneToOneFields and ManyToManyFields). In case it is ...-to-many field, this is existentially quantified. But here the ForeignKeys are many-to-one relations, so that does not matter.

Note: ForeignKeys to B (or C) is typically named b (or c), not a (or b), since that is the name of the current model. The name of a relation typically specifies how the object(s) it targets relate to the current model.

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