I have two models:
class A(models.Model):
name = models.CharField(max_length=100, unique=True)
class B(models.Model):
a = models.ForeignKey(A)
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()
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)