#27650: ForeignKey.validate asks for the wrong db_for_read
-------------------------------------+-------------------------------------
Reporter: Sven | Owner: nobody
Coenye |
Type: Bug | Status: new
Component: Database | Version: 1.10
layer (models, ORM) |
Severity: Normal | Keywords:
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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Use the following in the database router:
{{{
def db_for_read(self, model, **hints):
return settings.DATABASE_APPS_MAPPING.get(model._meta.app_label,
None)
}}}
with
{{{
DATABASE_APPS_MAPPING = {
'main_app': 'orders',
'support_app': 'legacy_db'}
}}}
and in main_app.models.py
{{{
from support_app.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) # AdProgram
resides in legacy_db
}}}
ForeignKey.validate() contains this code
{{{
using = router.db_for_read(model_instance.__class__,
instance=model_instance)
}}}
When submitting a ModelForm for the Order model (containing all three
fields), model_instance is the Order model for all fields, so in
db_for_read(), model._meta.app_label is "main_app" for all fields. During
the validation of the "program" field, this leads to the validation query
being submitted to the wrong database and the application terminates in an
uncaught ProgrammingError exception.
Changing the line above to
{{{
using = router.db_for_read(self.remote_field.model,
instance=model_instance)
}}}
passes the model of the target field instead. model._meta.app_label in
db_for_read() is now "support_app" and the "legacy_db" database is
returned. For target models in the same application as model_instance, the
field app_label will be the same as the model_instance app_label so the
correct database is retrieved in non-cross database cases as well.
I realize my particular database router code may have an influence on
this, but I couldn't see a way for a router to derive that it should
retrieve the "legacy_db" database with the information it is provided.
All Django tests appear to pass with the change in place on the
stable/1.10.x branch.
--
Ticket URL: <https://code.djangoproject.com/ticket/27650>
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