Filtering on multiple indirect relations to same table gives incorrect SQL

[Pavel Anossov]

I came across the following problem: if you filter through two foreign keys to same table, it gets referenced only once, like this:

class C(model):
    somefield = IntegerField()

# Every B is related to a C
class B(model):
    c = ForeignKey(C)

# Every A is related (differently) to two Bs, and indirectly to two different Cs
class A(model):
    b1 = ForeignKey(B, db_column='b1')
    b2 = ForeignKey(B, db_column='b2')

A.objects.filter(b1__c = 1).filter(b2__c = 2).query.as_sql()

# SELECT ...
# FROM `A`
# INNER JOIN `B` ON (`A`.`b1` = `B`.`id`)
# INNER JOIN `B` T4 ON (`A`.`b2` = T4.`id`)
# WHERE `B`.`c_id` = 1  AND `B`.`c_id` = 2
#
# This is wrong, it should be
# SELECT ...
# FROM `A`
# INNER JOIN `B` ON (`A`.`b1` = `B`.`id`)
# INNER JOIN `B` T4 ON (`A`.`b2` = T4.`id`)
# WHERE `B`.`c_id` = 1  AND `T4`.`c_id` = 2

# If we filter deeper, it gets more obvious:
A.objects.filter(b1__c__somefield = 1).filter(b2__c__somefield = 2).query.as_sql()

# SELECT ...
# FROM `A`
# INNER JOIN `B` ON (`A`.`b1` = `B`.`id`)
# INNER JOIN `C` ON (`B`.`c_id` = `C`.`id`)
# INNER JOIN `B` T4 ON (`A`.`b2` = T4.`id`)
# WHERE `C`.`somefield` = 1  AND `C`.`somefield` = 2
#
# This is wrong, should be
# SELECT ...
# FROM `A`
# INNER JOIN `B` ON (`A`.`b1` = `B`.`id`)
# INNER JOIN `C` ON (`B`.`c_id` = `C`.`id`)
# INNER JOIN `B` T4 ON (`A`.`b2` = T4.`id`)
# INNER JOIN `C` T5 ON (`T4`.`c_id` = `T5`.`id`)
# WHERE `C`.`somefield` = 1  AND `T5`.`somefield` = 2

Suggested solution is to call sql.query.join in sql.query.setup_joins with always_create=dupe_multis not only in m2m fields, but in o2o and m2o fields as well (see patch). I'm not sure if it's not breaking anything though, I should test it.

[Pavel Anossov]

Patch certainly breaks filters, so never mind it.. yet the problem exists.

[oyvind]

Same as #7277 , #7125

[Jacob]

I don't think this is related to #7277, but it is indeed a duplicate of #7125.