Django: query all items that have a foreign key point to them

Question

I have two models:

class A(models.Model):
    name = models.CharField(max_length=100, unique=True)

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


        

Answer 1

This worked for me:

a_qs = A.objects.filter(b__a__isnull=False).distinct()

It gives only the objects of A which has a B object pointing to it. Use a__isnull=False to check whether a ForeignKey exists. If it does, it can, by definition, only point to A. Duplicates are removed with distinct().

When a related_name parameter has been set, e.g.

class A(models.Model):
    name = models.CharField(max_length=100, unique=True)

class B(models.Model):
    a = models.ForeignKey(A, related_name='b_list')

this worked:

a_qs = A.objects.filter(b_list__a__isnull=False).distinct() 

Answer 2

You can do it like this:

a_qs = A.objects.filter(b = b) 

where b is an object of class B and the b= refers to the lowercase model name which you want to query the reverse relationship.

Read more on lookups that span relationships here. It covers how to do a reverse lookup on models' ForeignKey attributes

Edit:

If you are looking for all objects which do not have any ForeignKey objects pointing to them, you can use exclude and __isnull

a_qs = A.objects.exclude(b__isnull = True)