'Returns a googledatastore.Filter representation of the filter.
Args:
adapter: A datastore_rpc.AbstractAdapter'

def _to_pb_v1(self, adapter):

    filter_pb = googledatastore.Filter()
    prop_filter_pb = filter_pb.property_filter
    adapter.get_query_converter()._v3_filter_to_v1_property_filter(self._filter, prop_filter_pb)
    return filter_pb
