Chaining multiple filter() in Django, is this a bug?

Question

I always assumed that chaining multiple filter() calls in Django was always the same as collecting them in a single call.

# Equivalent
Model.objects.filter(foo=1).filter(bar=2)
Model.objects.filter(foo=1,bar=2)

but I have run across a complicated queryset in my code where this is not the case

class Inventory(models.Model):
    book = models.ForeignKey(Book)

class Profile(models.Model):
    user = models.OneToOneField(auth.models.User)
    vacation = models.BooleanField()
    country = models.CharField(max_length=30)

# Not Equivalent!
Book.objects.filter(inventory__user__profile__vacation=False).filter(inventory__user__profile__country='BR')
Book.objects.filter(inventory__user__profile__vacation=False, inventory__user__profile__country='BR')

The generated SQL is

SELECT "library_book"."id", "library_book"."asin", "library_book"."added", "library_book"."updated" FROM "library_book" INNER JOIN "library_inventory" ON ("library_book"."id" = "library_inventory"."book_id") INNER JOIN "auth_user" ON ("library_inventory"."user_id" = "auth_user"."id") INNER JOIN "library_profile" ON ("auth_user"."id" = "library_profile"."user_id") INNER JOIN "library_inventory" T5 ON ("library_book"."id" = T5."book_id") INNER JOIN "auth_user" T6 ON (T5."user_id" = T6."id") INNER JOIN "library_profile" T7 ON (T6."id" = T7."user_id") WHERE ("library_profile"."vacation" = False  AND T7."country" = BR )
SELECT "library_book"."id", "library_book"."asin", "library_book"."added", "library_book"."updated" FROM "library_book" INNER JOIN "library_inventory" ON ("library_book"."id" = "library_inventory"."book_id") INNER JOIN "auth_user" ON ("library_inventory"."user_id" = "auth_user"."id") INNER JOIN "library_profile" ON ("auth_user"."id" = "library_profile"."user_id") WHERE ("library_profile"."vacation" = False  AND "library_profile"."country" = BR )

The first queryset with the chained filter() calls joins the Inventory model twice effectively creating an OR between the two conditions whereas the second queryset ANDs the two conditions together. I was expecting that the first query would also AND the two conditions. Is this the expected behavior or is this a bug in Django?

The answer to a related question Is there a downside to using ".filter().filter().filter()..." in Django? seems to indicated that the two querysets should be equivalent.

Answer 1

The way I understand it is that they are subtly different by design (and I am certainly open for correction): filter(A, B) will first filter according to A and then subfilter according to B, while filter(A).filter(B) will return a row that matches A 'and' a potentially different row that matches B.

Look at the example here:

https://docs.djangoproject.com/en/dev/topics/db/queries/#spanning-multi-valued-relationships

particularly:

Everything inside a single filter() call is applied simultaneously to filter out items matching all those requirements. Successive filter() calls further restrict the set of objects

...

In this second example (filter(A).filter(B)), the first filter restricted the queryset to (A). The second filter restricted the set of blogs further to those that are also (B). The entries select by the second filter may or may not be the same as the entries in the first filter.`