Opened 3 years ago

Closed 3 years ago

Last modified 3 years ago

#20955 closed Bug (fixed)

Deep select_related to same model returns incorrect relationship

Reported by: Alex Hayes Owned by: nobody
Component: Database layer (models, ORM) Version: master
Severity: Release blocker Keywords:
Cc: Triage Stage: Accepted
Has patch: no Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no

Description

When using select_related on a deeply nested relationship and you have two (or possibly more) relationships that use the same model the SQL query is incorrect and thus the resulting model incorrect.

This is best illustrated with an example:

expect = Task.objects.get(pk=1)
actual = Task.objects.select_related('creator__staffuser__staff', 'owner__staffuser__staff').get(pk=1)

if actual.creator.staffuser.staff != expect.creator.staffuser.staff:
    print "Creator Incorrect"
if actual.owner.staffuser.staff != expect.owner.staffuser.staff:
    print "Owner Incorrect"

Outputs Owner Incorrect

Test Project

I've created a test project at https://github.com/alexhayes/deepselectrelated

This test project has two tests that illustrates the difference in behaviour.

Issue Location

It appears that this issue exists in the actual SQL statements that are being generated, as taken from my test project:

         SELECT "deepselectrelated_task"."id", "deepselectrelated_task"."title", "deepselectrelated_task"."creator_id", "deepselectrelated_task"."owner_id", 
                "auth_user"."id", "auth_user"."password", "auth_user"."last_login", "auth_user"."is_superuser", "auth_user"."username", "auth_user"."first_name", "auth_user"."last_name", "auth_user"."email", "auth_user"."is_staff", "auth_user"."is_active", "auth_user"."date_joined", 
                "deepselectrelated_staffuser"."user_ptr_id", "deepselectrelated_staffuser"."staff_id", 
                "deepselectrelated_staff"."id", "deepselectrelated_staff"."name", 
                T5."id", T5."password", T5."last_login", T5."is_superuser", T5."username", T5."first_name", T5."last_name", T5."email", T5."is_staff", T5."is_active", T5."date_joined", 
                T6."user_ptr_id", T6."staff_id", 
                "deepselectrelated_staff"."id", "deepselectrelated_staff"."name" 
           FROM "deepselectrelated_task" 
     INNER JOIN "auth_user" ON ( "deepselectrelated_task"."creator_id" = "auth_user"."id" ) 
LEFT OUTER JOIN "deepselectrelated_staffuser" ON ( "auth_user"."id" = "deepselectrelated_staffuser"."user_ptr_id" ) 
LEFT OUTER JOIN "deepselectrelated_staff" ON ( "deepselectrelated_staffuser"."staff_id" = "deepselectrelated_staff"."id" ) 
     INNER JOIN "auth_user" T5 ON ( "deepselectrelated_task"."owner_id" = T5."id" ) 
LEFT OUTER JOIN "deepselectrelated_staffuser" T6 ON ( T5."id" = T6."user_ptr_id" )

Note that there is only one join to the staff table and the following part of the SELECT query is incorrectly repeated:

"deepselectrelated_staff"."id", "deepselectrelated_staff"."name"

There should infact be two joins to the staff table and the SELECT should be as follows:

         SELECT "deepselectrelated_task"."id", "deepselectrelated_task"."title", "deepselectrelated_task"."creator_id", "deepselectrelated_task"."owner_id", 
                "auth_user"."id", "auth_user"."password", "auth_user"."last_login", "auth_user"."is_superuser", "auth_user"."username", "auth_user"."first_name", "auth_user"."last_name", "auth_user"."email", "auth_user"."is_staff", "auth_user"."is_active", "auth_user"."date_joined", 
                "deepselectrelated_staffuser"."user_ptr_id", "deepselectrelated_staffuser"."staff_id", 
                "deepselectrelated_staff"."id", "deepselectrelated_staff"."name", 
                T5."id", T5."password", T5."last_login", T5."is_superuser", T5."username", T5."first_name", T5."last_name", T5."email", T5."is_staff", T5."is_active", T5."date_joined", 
                T6."user_ptr_id", T6."staff_id", 
                T7."id", T7."name" 
           FROM "deepselectrelated_task" 
     INNER JOIN "auth_user" ON ( "deepselectrelated_task"."creator_id" = "auth_user"."id" ) 
LEFT OUTER JOIN "deepselectrelated_staffuser" ON ( "auth_user"."id" = "deepselectrelated_staffuser"."user_ptr_id" ) 
LEFT OUTER JOIN "deepselectrelated_staff" ON ( "deepselectrelated_staffuser"."staff_id" = "deepselectrelated_staff"."id" ) 
     INNER JOIN "auth_user" T5 ON ( "deepselectrelated_task"."owner_id" = T5."id" ) 
LEFT OUTER JOIN "deepselectrelated_staffuser" T6 ON ( T5."id" = T6."user_ptr_id" )
LEFT OUTER JOIN "deepselectrelated_staff" T7 ON ( T6."staff_id" = T7."id" )

Change History (3)

comment:1 Changed 3 years ago by Anssi Kääriäinen

Needs documentation: unset
Needs tests: unset
Patch needs improvement: unset
Severity: NormalRelease blocker
Triage Stage: UnreviewedAccepted

Seems to be 1.6.x regression, so release blocker. I will try to fix this today.

comment:2 Changed 3 years ago by Anssi Kääriäinen <akaariai@…>

Resolution: fixed
Status: newclosed

In 8d65b6082c8bf5df25608d8733470879a8a61d7d:

Fixed #20955 -- select_related regression

In cases where the same connection (from model A to model B along the
same field) was needed multiple times in a select_related query, the
join setup code mistakenly reused an existing join.

comment:3 Changed 3 years ago by Anssi Kääriäinen <akaariai@…>

In 161e26c2ec9f88bf0395941aaa2fd193b110affd:

[1.6.x] Fixed #20955 -- select_related regression

In cases where the same connection (from model A to model B along the
same field) was needed multiple times in a select_related query, the
join setup code mistakenly reused an existing join.

Backpatch of 8d65b6082c8bf5df25608d8733470879a8a61d7d.

Conflicts:

django/db/models/sql/compiler.py
tests/queries/tests.py

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