Work around Django 2.1 db router changes

Changes from django/django#9360 caused more aggressive router lookups, which resulted in infinite recursion when attempting to save some new related objects.

corehq/sql_db/models.py

@@ -114,6 +114,10 @@ def partition_attr(self):
 
     @property
     def partition_value(self):
+        if self.partition_attr not in self.__dict__ and self.pk is None:
+            # prevent infinite recursion in db router
+            raise AttributeError(
+                f"unknown partition value for unsaved {type(self).__name__}")
         return getattr(self, self.partition_attr)
 
     @property

corehq/sql_db/routers.py

@@ -166,7 +166,10 @@ def get_db_for_partitioned_model(model, hints):
         raise Exception(f'Unable to perform routing, multiple hints provided: {hints}')
 
     if HINT_INSTANCE in hints:
-        partition_value = getattr(hints[HINT_INSTANCE], 'partition_value', None)
+        instance = hints[HINT_INSTANCE]
+        if instance._state.db is not None:
+            return instance._state.db
+        partition_value = getattr(instance, 'partition_value', None)
         if partition_value is not None:
             return get_db_alias_for_partitioned_doc(partition_value)
     if hints.get(HINT_PLPROXY):