FilteredRelation join duplicated when referenced in multiple filter statements

[Ben Nace]

Given the following models

class Grandparent(models.Model):
    tag = models.CharField(max_length=100)
    enabled = models.BooleanField(default=True)


class Parent(models.Model):
    tag = models.CharField(max_length=100)
    grandparent = models.ForeignKey(Grandparent, on_delete=models.CASCADE)


class Child(models.Model):
    tag = models.CharField(max_length=100)
    parents = models.ManyToManyField(Parent)

When doing the following query with a filtered relation and a single filter statement, the SQL is generated as expected:

Child.objects.alias(a_parents=FilteredRelation('parents', condition=Q(tag='a'))).filter(a_parents__grandparent__enabled=True, a_parents__grandparent__tag='x')

SELECT "test_app_child"."id", "test_app_child"."tag" FROM "test_app_child" INNER JOIN "test_app_child_parents" ON ("test_app_child"."id" = "test_app_child_parents"."child_id") INNER JOIN "test_app_parent" a_parents ON ("test_app_child_parents"."parent_id" = a_parents."id" AND ("test_app_child"."tag" = 'a')) INNER JOIN "test_app_grandparent" ON (a_parents."grandparent_id" = "test_app_grandparent"."id") WHERE ("test_app_grandparent"."enabled" AND "test_app_grandparent"."tag" = 'x')

However, when the filter conditions are split across multiple calls to the filter function, say from them being applied in different functions, the joins for the filtered relation get duplicated in the SQL.

Child.objects.alias(a_parents=FilteredRelation('parents', condition=Q(tag='a'))).filter(a_parents__isnull=False).filter(a_parents__grandparent__tag='x')

SELECT "test_app_child"."id", "test_app_child"."tag" FROM "test_app_child" INNER JOIN "test_app_child_parents" ON ("test_app_child"."id" = "test_app_child_parents"."child_id") INNER JOIN "test_app_parent" a_parents ON ("test_app_child_parents"."parent_id" = a_parents."id" AND ("test_app_child"."tag" = 'a')) INNER JOIN "test_app_child_parents" T4 ON ("test_app_child"."id" = T4."child_id") INNER JOIN "test_app_parent" T5 ON (T4."parent_id" = T5."id" AND ("test_app_child"."tag" = 'a')) INNER JOIN "test_app_grandparent" ON (T5."grandparent_id" = "test_app_grandparent"."id") WHERE (a_parents."id" IS NOT NULL AND "test_app_grandparent"."tag" = 'x')

[Mariusz Felisiak]

This is expected filter(A, B) and filter(A).filter(B) don't behave the same when multi-valued relationships are involved ​as ​documented.