#17002: ManyToManyField through a model which extends some other model
-------------------------------------+-------------------------------------
Reporter: mitar | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: SVN
(models, ORM) | Resolution:
Severity: Normal | Triage Stage: Design
Keywords: | decision needed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Changes (by sebastian):
* needs_better_patch: => 0
* needs_tests: => 0
* version: 1.3 => SVN
* needs_docs: => 0
* type: New feature => Bug
* stage: Unreviewed => Design decision needed
Comment:
Django expects the foreign key attributes to be present in the
intermediary model given as the `through=` argument (and not some non-
abstract parent) when constructing the related queries. On the other hand,
model validation and table creation works even when the foreign key
attributes are in the base class, or split over base and derived class as
in e.g.
{{{
class A(models.Model):
pass
class B(models.Model):
a_s = models.ManyToManyField(A, through='Derived')
class Base(models.Model):
a = models.ForeignKey(A)
class Derived(Base):
b = models.ForeignKey(B)
}}}
The example validates just fine and even generates the appropriate
database structure, but related queries fail during runtime, e.g.
`B.objects.get(pk=1).a_s.all()` throws "DatabaseError: no such column:
app_derived.a_id".
Can you give an example where foreign keys in the base class of the
intermediary model (as opposed to arbitrary attributes, or methods) are
necessary or would be useful?
In any case, I think the behavior here should be consistent. If the query
cannot be created (because attributes are expected to be present in the
derived table), then the validation of the model should not succeed. Or
otherwise, the query should take attributes imported from base classes
into account and insert the necessary joins.
--
Ticket URL: <https://code.djangoproject.com/ticket/17002#comment:1>
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