Calling annotate with a case statement that references a recursive many to many relationship creates duplicates.

[David Cooke]

So I have a Challenge model that has an unlocks field which is a recursive, asymmetric many to many field with a reverse name of unlocked_by. I call annotate on it to add an "unlocked" field which checks if the challenges unlocked_by contains any ids in a list of solved challenges, doing this inserts all the solved challenges into the queryset that annotate is being called on. There's an example of this here: ​https://github.com/david-cooke/djangobug/blob/master/bug/tests.py

The sql being run is

SELECT "bug_challenge"."id", "bug_challenge"."name", CASE WHEN "bug_challenge_unlocks"."from_challenge_id" IN (5, 1) THEN True ELSE False END AS "unlocked" FROM "bug_challenge" LEFT OUTER JOIN "bug_challenge_unlocks" ON ("bug_challenge"."id" = "bug_challenge_unlocks"."to_challenge_id")

so it seems like the left outer join is adding the duplicates although thats needed for the case statement.

It is possible to work around this by calling value_list(flat=True).distinct('pk') however isn't compatible with more complicated examples such as ​https://github.com/ractf/core/blob/master/challenge/views.py#L50

[Mariusz Felisiak]

This is documented behavior (see the warning in the ​QuerySet.values() docs):

Because ManyToManyField attributes and reverse relations can have multiple related rows, including these can have a multiplier effect on the size of your result set. This will be especially pronounced if you include multiple such fields in your values() query, in which case all possible combinations will be returned.

You can use Subquery() or .distinct(...) to avoid duplicate.

[Simon Charette]

The ORM is able to optimize .filter(unlocked_by__in=solved_challenges) into an INNER JOIN but it cannot do the same for annotations because the could be doing some special handling of NULL and it would require some pretty sophisticated introspection to determine it's safe to do. It would also be backward incompatible at this point. You should rely on Subquery or even Exists in your reported case instead.