id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
27629	Inconsistent check of allow_relation in ForwardManyToOneDescriptor.__set__	Sven Coenye	Stefan R. Filipek	"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?"	Bug	closed	Database layer (models, ORM)	1.11	Normal	fixed	allow_relation ForwardManyToOneDescriptor		Accepted	1	0	0	0	0	0