[Django] #18375: F() doesn't work as expected across relations

[Django]

#18375: F() doesn't work as expected across relations
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     Reporter:  FunkyBob                      |      Owner:  nobody
         Type:  Uncategorized                 |     Status:  new
    Component:  Database layer (models, ORM)  |    Version:  1.4
     Severity:  Normal                        |   Keywords:
 Triage Stage:  Unreviewed                    |  Has patch:  0
Easy pickings:  0                             |      UI/UX:  0
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 When trying to compare a field to another on the same related record, F()
 will compare against a separate alias of the table, thus not guaranteeing
 filtering against the same row.

 There doesn't appear to be anything in the docs or tests to indicate what
 the 'correct' behavior is.  Also, there's no apparent way to control it.

 Using the attached models, I get the following:

 {{{

 (InteractiveConsole)
 >>> from sample import models
 >>> from django.db.models import Q, F
 >>> qset =
 models.Uber.objects.filter(unter__available__gt=F('unter__used'))
 >>> str(qset.query)
 'SELECT "sample_uber"."id", "sample_uber"."title" FROM "sample_uber" INNER
 JOIN "sample_unter" ON ("sample_uber"."id" = "sample_unter"."uber_id")
 INNER JOIN "sample_unter" T3 ON ("sample_uber"."id" = T3."uber_id") WHERE
 T3."available" >  "sample_unter"."used"'
 }}}

 The SQL nicely formatted shows:
 {{{
 SELECT "sample_uber"."id", "sample_uber"."title"
 FROM "sample_uber"
 INNER JOIN "sample_unter" ON ("sample_uber"."id" =
 "sample_unter"."uber_id")
 INNER JOIN "sample_unter" T3 ON ("sample_uber"."id" = T3."uber_id")
 WHERE T3."available" >  "sample_unter"."used"
 }}}

 So, the F('unter__used') is using the 'sample_unter' join, whereas the
 unter__available__gt is using the T3 alias.

-- 
Ticket URL: <https://code.djangoproject.com/ticket/18375>
Django <https://code.djangoproject.com/>
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