Hello, I have a two models (User, Project) in m2m relationship with intermediate (Role) table.
When I’m selecting all users in project and I also want to select corresponding
role, I simply annotate one field from Role table using F expression:
Users.objects.all().filter(projects__name=‘Django’).annotate(role=F(‘roles__role’))
Annotate in this case doesn’t create new join, because `roles` table is already
joined to filter on data from `projects` table. This works well.
However, I tried to same in `prefetch_related` and I’m getting duplicate rows,
because there’s a new JOIN statement added. (Usecase: Selecting all projects in
DB with all users per project)
The SQL statement with `prefetch_related`, but without `annotate` looks like
this:
roles = Prefetch(
‘users',
queryset=User.objects.all()
)
qs = Project.objects.prefetch_related(roles)
SELECT
("users_role"."project_id") AS "_prefetch_related_val_project_id",
— other fields here
FROM "users_user"
INNER JOIN "users_role" ON ("users_user"."id" = "users_role"."user_id")
WHERE "users_role"."project_id” IN (1, 2, 3, 4, 5)
As you can see, the table `users_role` is already joined, so I’m basically
looking for Django ORM expression which generates following SQL query:
SELECT
("users_role"."project_id") AS "_prefetch_related_val_project_id”,
“users_role_.”role”,
— other fields here
FROM "users_user"
INNER JOIN "users_role" ON ("users_user"."id" = "users_role"."user_id")
WHERE "users_role"."project_id” IN (1, 2, 3, 4, 5)
Unfortunatelly, following expression generates incorrect SQL:
roles = Prefetch(
‘users',
queryset=User.objects.all().annotate(role=F('roles__role'))
)
qs = Project.objects.prefetch_related(roles)
SELECT
("users_role"."project_id") AS "_prefetch_related_val_project_id",
"users_role"."role" AS “role”,
— other fields here
FROM "users_user"
LEFT OUTER JOIN "users_role" ON ("users_user"."id" = "users_role"."user_id")
INNER JOIN "users_role" T3 ON ("users_user"."id" = T3."user_id")
WHERE T3."project_id" IN
(1, 2, 3, 4, 5)
The extra `left outer join` causes duplicate entries.
I’ve found one ticket (https://code.djangoproject.com/ticket/27144
<https://code.djangoproject.com/ticket/27144>) which seems to be relevant, but
it’s old and closed.
Any ideas? Is it bug or is there really a reason to include extra JOIN? I’m not
very skilled in relational algebra.
Thank you in advance!
Cheers,
Tom
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