Assume composite PK when fetching instance for master view

i.e. stop trying a simple get() which would assume not only a simple PK, but
also assumes the PK is same as defined by the class mapper.  in some cases it
may be helpful to use a different PK from what mapper defines

tailbone/views/master.py

@@ -3658,24 +3658,19 @@ class MasterView(View):
         doing a database lookup.  If the instance cannot be found, raises 404.
         """
         model_keys = self.get_model_key(as_tuple=True)
-
-        # if just one primary key, simple get() will work
-        if len(model_keys) == 1:
-            model_key = model_keys[0]
-            key = self.request.matchdict[model_key]
-
-            obj = self.Session.query(self.get_model_class()).get(key)
-            if not obj:
-                raise self.notfound()
-
-        else: # composite key; fetch accordingly
-            # TODO: should perhaps use filter() instead of get() here?
         query = self.Session.query(self.get_model_class())
-            for i, model_key in enumerate(model_keys):
+
+        def filtr(query, model_key):
             key = self.request.matchdict[model_key]
             if self.key_is_integer(model_key):
                 key = int(key)
             query = query.filter(getattr(self.model_class, model_key) == key)
+            return query
+
+        # filter query by composite key.  we use filter() instead of a simple
+        # get() here in case view uses a "pseudo-PK"
+        for i, model_key in enumerate(model_keys):
+            query = filtr(query, model_key)
         try:
             obj = query.one()
         except orm.exc.NoResultFound: