Opened 4 years ago

Last modified 6 months ago

#17461 assigned Cleanup/optimization

Document presumed order of foreign keys on intermediate M2M model

Reported by: flytwokites@… Owned by: Oxylo
Component: Documentation Version: 1.3
Severity: Normal Keywords:
Cc: Triage Stage: Accepted
Has patch: no Needs documentation: yes
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no

Description

When defining a many-to-many relationship from a model to itself and using an intermediary model,

class User(models.Model):
    name = models.CharField(max_length=100)
    
    followers = models.ManyToManyField('self', through='Relationship', symmetrical=False)

    def __unicode__(self):
        return self.name

class Relationship(models.Model):
    target = models.ForeignKey('User', related_name='r1')
    follower = models.ForeignKey('User', related_name='r2')
    created_at = models.DateTimeField(auto_now_add=True)

It seems that django determine 'from field' and 'to field' by it's definition order,
the first field always used as 'from field' and the second field always used as 'to field'.
So I MUST put target field defintion above the follower field.

I checked the document but can not found any infomation to confirm it,
I think the document should clearly explained the rule.

Change History (3)

comment:1 Changed 4 years ago by gabrielhurley

  • Needs documentation set
  • Needs tests unset
  • Patch needs improvement unset
  • Summary changed from Need more document for defining a many-to-many relationship from a model to itself and using an intermediary model to Document presumed order of foreign keys on intermediate M2M model
  • Triage Stage changed from Unreviewed to Accepted
  • Type changed from Uncategorized to Cleanup/optimization

This is true specifically for asymmetric M2M's using an intermediate model; it's an implementation detail coded into the accessor calls here:

https://code.djangoproject.com/browser/django/trunk/django/db/models/fields/related.py#L1135

and a little lower down, here:

https://code.djangoproject.com/browser/django/trunk/django/db/models/fields/related.py#L1154

It's worth noting that this the case in the docs, probably under the "symmetrical" section of the docs here:

https://docs.djangoproject.com/en/1.3/ref/models/fields/#django.db.models.ManyToManyField.symmetrical

comment:2 Changed 6 months ago by Oxylo

  • Owner changed from nobody to Oxylo
  • Status changed from new to assigned

comment:3 Changed 6 months ago by Oxylo

Confirmed today on master that Django still determines the 'from field' and the 'to field' by its definition order in this case. We proved this by comparing the SQL generated for both orderings of the fields "target" and "follower"

When the field "target" is above the field "follower"

u1 = User.objects.create(name="Erik")
print u1.followers.all().query

returns the desired SQL:

SELECT "testm2m_user"."id", "testm2m_user"."name"
FROM "testm2m_user" INNER JOIN "testm2m_relationship"
ON ( "testm2m_user"."id" = "testm2m_relationship"."follower_id" )
WHERE "testm2m_relationship"."target_id" = 1

but if the field "follower" is above the field "target" the following (incorrect) SQL is returned:

SELECT "testm2m_user"."id", "testm2m_user"."name"
FROM "testm2m_user" INNER JOIN "testm2m_relationship"
ON ( "testm2m_user"."id" = "testm2m_relationship"."target_id" )
WHERE "testm2m_relationship"."follower_id" = 1

Conclusion:
This issue still persists and needs to be documented accordingly.

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