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

django/db/models/fields/related.py

@@ -228,8 +228,7 @@ class SingleRelatedObjectDescriptor(six.with_metaclass(RenameRelatedObjectDescri
                 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": instance is on database "%s", value is on database "%s"' %
-                                        (value, instance._state.db, value._state.db))
+                    raise ValueError('Cannot assign "%r": the current database router prevents this relation.' % value)
 
         related_pk = tuple([getattr(instance, field.attname) for field in self.related.field.foreign_related_fields])
         if None in related_pk:
@@ -328,8 +327,7 @@ class ReverseSingleRelatedObjectDescriptor(six.with_metaclass(RenameRelatedObjec
                 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": instance is on database "%s", value is on database "%s"' %
-                                        (value, instance._state.db, value._state.db))
+                    raise ValueError('Cannot assign "%r": the current database router prevents this relation.' % value)
 
         # If we're setting the value of a OneToOneField to None, we need to clear
         # out the cache on any old related object. Otherwise, deleting the