Implicit related objects filtration because of the set Manager.

[ApaDoctor]

When abstract model is defined with objects manager.
And its child-model defines another manager - RelatedManager uses defined manager instead of objects.

class SomeAbstractModel(models.Model):
    class Meta:
        abstract = True

    objects = models.Manager()


class CustomManager(models.Manager):
    def get_queryset(self):
        return super().get_queryset().filter(some_field=True)


class SomeThing(SomeAbstractModel):
    some_field = models.BooleanField()

    parent = models.ForeignKey(
        "SomeThing", related_name="children", on_delete=models.SET_NULL, null=True
    )

    custom_objects = CustomManager()

So we have abstract model, which defines objects manager for the model.

Our model uses that abstract class.

I created some objects:

SomeThing(some_field=True).save()
SomeThing.objects.bulk_create([SomeThing(some_field=bool(x%2), parent_id=1) for x in range(0,6)])

After i reloaded shell i tried to make queries:

>>> x = SomeThing.objects.get(id=1)
>>> x.children.count()
3
>>>SomeThing.objects.filter(parent_id=1).count()
6

Generated raw SQL for first query is:

SELECT COUNT(*) AS "__count" FROM "sapp_something" WHERE ("sapp_something"."some_field" = true AND "some_something"."parent_id" = 1)

[Mariusz Felisiak]

Thanks for the report, IMO everything works properly, let's analyze this step by step.

You inherited from SomeAbstractModel with a manager and define a custom_manager in SomeThing. custom_manager is a default manager for SomeThing that's why it's used by related manager (e.g. x.children). In the same time SomeThing inherited objects manager from SomeAbstractModel (it's no longer a default manager provided by Django, see ​manager-names) so when you call it explicit e.g. SomeThing.objects.filter(parent_id=1).count() it returns all rows, without an extra filter (see also ​custom-managers-and-model-inheritance and ​using-a-custom-reverse-manager).