ValueError when database router prevents relation is unclear

[Dan Loewenherz]

I'm using a custom database router to manage queries between two databases.

In my router, the allow_relation method was set incorrectly, and as a result, two models from separate apps weren't allowed to reference each other. The ValueError I received looked like this:

Cannot assign "<ContentType: xxx>": instance is on database "default", value is on database "default

I opened up the source that raised this error, and saw this line:

if not router.allow_relation(value, instance):
    raise ValueError('Cannot assign "%r": instance is on database "%s", value is on database "%s"' % (value, instance._state.db, value._state.db))

The databases that these objects are on had nothing to do with how my router was routing the query, and kind of led me on a wild goose chase until I opened up the source. I would suggest a copy change to something like: "Cannot assign "%r": the current database router prevents this relation."

[anonymous]

PR @ ​https://github.com/django/django/pull/1035

[Simon Charette]

The exception message re-wording makes sense.

[Tim Graham <timograham@…>]

In 07a73a2714a504ae487112c1679977fccc19b159:

Fixed #20337 -- Clarified error message when database relation is not allowed.