#16715: Wrong JOIN with nested null-able foreign keys
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Reporter: sebastian | Owner: nobody
Type: Bug | Status: new
Milestone: | Component: Database layer
Version: SVN | (models, ORM)
Resolution: | Severity: Normal
Triage Stage: Accepted | Keywords: join, values,
Needs documentation: 0 | nested, foreign key, null-able
Patch needs improvement: 1 | Has patch: 1
UI/UX: 0 | Needs tests: 1
| Easy pickings: 0
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Comment (by sebastian):
I am attaching a patch that adds another test to
`regressiontests/model_inheritance_select_related`. In this test, very
similar to what I had in comment:2, I basically have the following models:
{{{
class Chain(models.Model):
pass
class Place(models.Model):
pass
class Franchise(Place):
chain = models.ForeignKey(Chain)
}}}
(That is, one multi-table inheritance from Place to Franchise, and one
non-nullable foreign key from Franchise to Chain.)
I then create a Place object explicitly (i.e. a place that is not a
franchise), and a Franchise object (implicitly creating a Place object)
with an associated Chain object. The following three queries now should
all return the same queryset consisting of the two Place objects (one for
the Place, one for the Franchise):
{{{
Place.objects.all()
Place.objects.select_related("franchise")
Place.objects.select_related("franchise__chain")
}}}
However, due to the wrong join being selected between the Franchise and
Chain model, the third queryset only returns the Place that is also a
Franchise. In other words, the other Place that was created explicitly
without a Franchise is now missing.
When adding an attribute `null=True` to the chain foreign key in
Franchise, all three querysets return both objects. This is because
`null=True` forces the correct left outer join between Franchise and
Chain.
Regarding the issue with `values` and foreign keys (instead of the multi-
table inheritance/one-to-one relation used in this test), I guess both
problems have the some original cause, so I didn't write a test for
`values` (neither for foreign-key instead of one-to-one relation). Fixing
the `select_related` issue might also fix the other problem(s). My guess
is that something deep inside the query generator goes wrong when it makes
the choice of inner join vs. left outer join on nested foreign-key
relationships.
--
Ticket URL: <https://code.djangoproject.com/ticket/16715#comment:8>
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