clarify that attr_name is a string for [ticket:2176]

diff --git a/lib/sqlalchemy/orm/collections.py b/lib/sqlalchemy/orm/collections.py
index 142519206cf1a2941829b6f7d9110aef9dc71224..7ac70fe5a4fe3ba56cd2d877d12b065a1e7486a3 100644 (file)
--- a/lib/sqlalchemy/orm/collections.py
+++ b/lib/sqlalchemy/orm/collections.py
@@ -154,7 +154,8 @@ def attribute_mapped_collection(attr_name):
     """A dictionary-based collection type with attribute-based keying.
 
     Returns a MappedCollection factory with a keying based on the
-    'attr_name' attribute of entities in the collection.
+    'attr_name' attribute of entities in the collection, where ``attr_name``
+    is the string name of the attribute.
 
     The key value must be immutable for the lifetime of the object.  You
     can not, for example, map on foreign key values if those key values will