Question

To make my question more understandable, here’s an example.

There are two models:

class A(models.Model):
    name = models.CharField(max_length = 10)

class B(models.Model):
    a = models.ForeignKey(A)
    name = models.CharField(max_length = 10)

So in this example, A and B are of one-to-many relationship. Now let’s say I’d like to make the following query: find an A that has at least one B as child. In sql, clearly I should use exists clause. Is it possible to achieve exactly the same with orm?

I’ve done some research on that but can’t find a perfect match to the sql query. The closest solution is like:

A.objects.filter(b__pk__gt = 0).distinct()

But it’s still far from the exists clause in sql and might not be as efficient as exists.

Answer 1

The following will select all As that have one or more associated Bs:

A.objects.filter(b__isnull=False)

Switching it to b__isnull=True will select only As that have no Bs associated with them.