#30389: Duplicate object when ordering through a foreign key
-------------------------------------+-------------------------------------
Reporter: Ajabep | Owner: nobody
Type: Bug | Status: new
Component: Database | Version:
layer (models, ORM) | Keywords: ordering, foreign
Severity: Normal | key
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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When we are using an ordering through a foreign key, a same object can be
resolved several times.
=== PoC
==== Models
{{{
#!python
class Team(models.Model):
name = models.CharField(max_length=255, primary_key=True)
class Meta:
ordering = ['-persons__creationtime']
class Person(models.Model):
uuid = models.UUIDField(primary_key=True, default=uuid.uuid4,
editable=False)
creationtime = models.DateTimeField(auto_now_add=True)
team = models.ForeignKey(Team, on_delete=models.CASCADE)
class Meta:
default_related_name = 'persons'
}}}
==== Query
`Team.objects.filter(name="R&D")`
==== Buggy result
`<QuerySet [<Team: Team object (R&D)>, <Team: Team object (R&D)>]>`
The same team is selected 2 times.
==== Expected result
`<QuerySet [<Team: Team object (R&D)>]>`
Each teams (here, only 1) only 1 time.
==== Small analysis
By dumping the SQL request, we observe that the ordering is translated by
a join instruction.
`SELECT "poc_team"."name" FROM "poc_team" LEFT OUTER JOIN "poc_person" ON
("poc_team"."name" = "poc_person"."team_id") WHERE "poc_team"."name" = R&D
ORDER BY "poc_person"."creationtime" DESC`
Thus, if a `Team` object is linked to two `Person` objects, the `Team`
will be selected 2 times. If it is linked to 3 `Person`, the `Team` will
be selected 3 times.
This bug occurred also when you are using listing some teams, joined by a
ManyToMany relation.
=== Workaround, waiting a fix
To avoid this bug, while there is no official fix, use the `distinct()`
method:
`Team.objects.filter(name="R&D").distinct()`
--
Ticket URL: <https://code.djangoproject.com/ticket/30389>
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