Suggestion: When a user connects two entities (creating an association) or when a user adds an association to an entity, display the properties so the user does not have to manually invoke them afterwards.
AND add a check box to the properties page that would auto-populate the newly generated association attribute in the entity - possibly in both entities. The system already adds the attributes to the entity (or entities). When the box is checked and the user clicks OK the system executes the necessary looping logic to populate the attribute(s).