prefetch_refprops

def prefetch_refprops(entities, *props):
    fields = [(entity, prop) for entity in entities for prop in props]
    ref_keys = [prop.get_value_for_datastore(x) for x, prop in fields]

    # Because if ref_keys has a None value,                                                                                                                                                            
    # db.get raise a BadArgumentError exception.                                                                                                                                                       
    for (i, v) in enumerate (ref_keys):
        if v is None:
            del ref_keys[i]
            del fields[i]

    ref_entities = dict((x.key(), x) for x in db.get(set(ref_keys)))
    for (entity, prop), ref_key in zip(fields, ref_keys):
        prop.__set__(entity, ref_entities[ref_key])

    return entities