A filter query returns more items than the original queryset provides after applying INNER JOIN

[Ľuboš Mjachky]

In our project, we identified that the filter query returns more entries than the number of entries stored in the initial queryset.

The following piece of code is involved:

# qs.count() == 4
scoped_repos = repo_viewset.get_queryset().values_list("pk", flat=True)
filtered_content = qs.filter(repositories__in=scoped_repos)
# filtered_content.count() == 8

The generated query:

SELECT * FROM "rpm_package" INNER JOIN "core_content" ON ("rpm_package"."content_ptr_id" = "core_content"."pulp_id") 
INNER JOIN "core_repositorycontent" ON ("core_content"."pulp_id" = "core_repositorycontent"."content_id") 
WHERE "core_repositorycontent"."repository_id" IN (c35b7039-2c2c-48e3-8f4f-b0eeabad8af1, ee39a78b-9dd5-4bdf-85d9-eb6406b6ef49)

One of the things being noticed is that the query is constructed with an INNER JOIN clause instead of a LEFT JOIN clause. The core_repositorycontent table contains a lot of duplicates. We believe that this should not be a problem. Adding the distinct() query at the end of the call resolves the issue. See ​https://github.com/pulp/pulpcore/pull/3642.

The question is whether this is a bug in Django (i.e., a filter query can return more elements than there are in the original queryset) or on our side, and we should restructure the query in a specific way. Any advice is welcome.

[Mariusz Felisiak]

This is a ​documented behavior. You can use Subquery() to prevent duplicates.