Deep select_related to same model returns incorrect relationship

[Alex Hayes]

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" )

[Anssi Kääriäinen]

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

[Anssi Kääriäinen <akaariai@…>]

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.

[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