id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
30389	Duplicate object when ordering through a foreign key	Ajabep	nobody	"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()`"	Bug	closed	Database layer (models, ORM)		Normal	invalid	ordering, foreign key		Unreviewed	0	0	0	0	0	0