def _combined_grouping_values(grouping_name, collection_a, collection_b):
    """
    returns a dict with values from both collections for a given grouping name

    Warning: collection2 overrides collection1 if there is a group_key conflict
    """
    new_grouping = collection_a.groupings.get(grouping_name, {}).copy()
    new_grouping.update(collection_b.groupings.get(grouping_name, {}))
    return new_grouping
