Django how to annotate manytomany field with count - django

Question

Considger the following:
class ModelA(models.Model):
pass
class ModelB(models.Model):
model_as = models.ManyToManyField(ModelA, db_index=True, related_name="model_bs", blank=True)
model_bs = model_a_object.model_bs.all()
for model_b in model_bs:
print(model_b.model_as.count())
3
2
So far so good. But I want to create an ordered list of model_bs depending on the count of model_as. My understanding is that simply this should do the trick:
model_bs = model_a_object.model_bs.all().annotate(count=Count("model_as")).order_by('count')
For some reason that doesn't work. When I print to check, the annotated count is wrong!!
for model_b in model_bs:
print(model_b.model_as.count(), model_b.count)
3 1
2 1
What did I do wrong? Here is the sql output for print(model_bs.query):
SELECT "mobile_modelb"."id",
"mobile_modelb"."modelb",
COUNT(DISTINCT "mobile_modelb_model_as"."modela_id") AS "count"
FROM "mobile_modelb"
INNER JOIN "mobile_modelb_model_as" ON ("mobile_modelb"."id" = "mobile_modelb_model_as"."modelb_id")
WHERE "mobile_modelb_model_as"."modela_id" = 7
GROUP BY "mobile_modelb"."id"
ORDER BY "count" ASC

Answer 1

Try adding distinct=True to the Count.
model_bs = model_a_object.model_bs.all().annotate(
count=Count("model_as", distinct=True)
).order_by('count')
EDIT:
The problem is because you're using model_a_object.model_bs.all(), you generate this WHERE clause:
WHERE "mobile_modelb_model_as"."modela_id" = 7
The solution may be to do:
ModelB.objects.annotate(
count=Count("model_as")
).filter(model_as=model_object_a)
If that doesn't work, try using values() before filter() to define the GROUP BY correctly.
ModelB.objects.annotate(
count=Count("model_as")
).values('id', 'count').filter(model_as=model_object_a)