#27629: Inconsistent check of allow_relation in
ForwardManyToOneDescriptor.__set__
-------------------------------------+-------------------------------------
Reporter: Sven | Owner: nobody
Coenye |
Type: Bug | Status: new
Component: Database | Version: 1.10
layer (models, ORM) | Keywords: allow_relation
Severity: Normal | ForwardManyToOneDescriptor
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
-------------------------------------+-------------------------------------
The method in django/db/models/fields/related_descriptors.py currently
contains the following code:
{{{
if instance._state.db is None:
instance._state.db = router.db_for_write(instance.__class__,
instance=value)
elif value._state.db is None:
value._state.db = router.db_for_write(value.__class__,
instance=instance)
elif value._state.db is not None and instance._state.db is not None:
if not router.allow_relation(value, instance):
raise ValueError('Cannot assign "%r": the current database router
prevents this relation.' % value)
}}}
When using an application containing the following model
{{{
from otherapp.models import AdProgram
class Order(models.Model):
first_name = models.CharField(max_length = 15)
last_name = models.CharField(max_length = 20)
program = models.ForeignKey(AdProgram, db_constraint=False) #
No cross database FK constraints under MSSQL
}}}
where the imported model belongs to a legacy database different from the
application's own database, the code in question does not flag an
inappropriate relationship. E.g. when submitting a ModelForm containing
these three fields, the value for program will be saved to the
application's database even if allow_relation would return False.
When the model definition is changed to
{{{
from otherapp.models import AdProgram, AdTerm
class Order(models.Model):
first_name = models.CharField(max_length = 15)
last_name = models.CharField(max_length = 20)
program = models.ForeignKey(AdProgram, db_constraint=False) #
No cross database FK constraints under MSSQL
term = models.ForeignKey(AdTerm, db_constraint=False)
}}}
"program" again gets a free pass, but "term" triggers the ValueError
exception.
The reason is that instance._state.db is None for the "program" field but
contains the value retrieved for "program" for any subsequent fields.
I believe those "elif" statements should be plain "if" statements so the
uninitialized value on the first pass does not fall through?
--
Ticket URL: <https://code.djangoproject.com/ticket/27629>
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