#30976: ManyToManyField does not create JOIN when target object contains a
ForeignKey with the same name as the default reverse_query_name
-------------------------------------+-------------------------------------
Reporter: Ben Davis | Owner: nobody
Type: Bug | Status: new
Component: Database | Version: 2.2
layer (models, ORM) |
Severity: Normal | Keywords:
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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Consider the following models. There is an `Entity` model which can be one
of two types, a USER or a GROUP. There are `User` and `UserGroup` models
that represent the sub-types, each having a `OneToOneField` pointing back
to `Entity` (basically model inheritance w/o subclassing).
An entity (which could be a user or a group) can exist another group (so,
a user can belong to a group, and a group can also belong to a group)
{{{
#!python
class Entity(models.Model):
entity_id = models.AutoField(primary_key=True)
name = models.CharField(max_length=128)
type = models.CharField(max_length=32, choices=(
('USER', 'User'),
('USER_GROUP', 'User group')
))
groups = models.ManyToManyField(
'UserGroup',
through='UserGroupMember',
through_fields=('member_entity', 'user_group')
)
class Meta:
managed = False
db_table = 'entity'
unique_together = (('type', 'name'),)
class User(models.Model):
user_id = models.AutoField(primary_key=True)
entity = models.OneToOneField(Entity, models.DO_NOTHING, unique=True)
email = models.EmailField()
class Meta:
managed = False
db_table = 'user'
class UserGroup(models.Model):
user_group_id = models.AutoField(primary_key=True)
entity = models.OneToOneField(Entity, models.DO_NOTHING, unique=True,
related_name='group')
disabled = models.BooleanField()
class Meta:
managed = False
db_table = 'user_group'
def __str__(self):
return self.entity.name
class UserGroupMember(models.Model):
user_group = models.ForeignKey(UserGroup, models.DO_NOTHING)
member_entity = models.ForeignKey(Entity, models.DO_NOTHING)
class Meta:
managed = False
db_table = 'user_group_member'
unique_together = (('user_group', 'member_entity'),)
def __str__(self):
print(f'{self.user_group}->{self.member_entity}')
}}}
When I make the following query, it does not produce the expected join:
{{{
>>> entity = Entity.objects.get(pk=2)
>>> print(entity.groups.all().query)
SELECT "user_group"."user_group_id", "user_group"."entity_id",
"user_group"."disabled" FROM "user_group" WHERE "user_group"."entity_id" =
2
}}}
This happens because the field accessor creates this implicit filter from
the target model: `UserGroup.objects.filter(entity__entity_id=2)`. It
assumes the default reverse name, which happens to conflict with the
`entity` field on the target model.
A workaround is to simply supply the `reverse_query_name` argument on the
`ManyToManyField` with something other than `"entity"`. However, it took
me quite a while to figure out what was happening here.
Django already warns about conflicting reverse names. It seems that there
should also be a warning for this case.
--
Ticket URL: <https://code.djangoproject.com/ticket/30976>
Django <https://code.djangoproject.com/>
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