I’m getting trouble with annotate in Django.

I have two models in projects. (just structure)

model A (field:name(char))
model B (field:A(A; related_name="b"), user(user), content(char; null=True))

The data of model B is just like…

row1 -> A:1, user:2, content:"content1"
row2 -> A:1, user:3, content:"content2"
row3 -> A:1, user:2, content:None
row4 -> A:2, user:1, content:None
row5 -> A:1, user:3, content:None

My intention is a unique value that combines user and content according to the pk of model A being searched.

If I look up pk:1 of Model A and the answer I want is 2, and the reason is as follows.

row1 -> A:1, user:2
row2 -> A:1, user:3

So the orginal code was…

A.objects.annotate(count=Count(Concat("b__user", "b__content"), distinct=True)).get(pk=pk)

However, a value error occurs when looking up model A that is not referenced in model B. (count:1)

What can should do?