Opened 13 years ago
Last modified 2 weeks ago
#17461 assigned Cleanup/optimization
Document presumed order of foreign keys on intermediate M2M model
Reported by: | Owned by: | Clifford Gama | |
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Component: | Documentation | Version: | 1.3 |
Severity: | Normal | Keywords: | |
Cc: | Triage Stage: | Accepted | |
Has patch: | yes | Needs documentation: | no |
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 by , 13 years ago
Needs documentation: | set |
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Summary: | Need more document for defining a many-to-many relationship from a model to itself and using an intermediary model → Document presumed order of foreign keys on intermediate M2M model |
Triage Stage: | Unreviewed → Accepted |
Type: | Uncategorized → Cleanup/optimization |
comment:3 by , 10 years ago
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.
comment:4 by , 2 weeks ago
Has patch: | set |
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Needs documentation: | unset |
Owner: | changed from | to
Status: | new → assigned |
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