def _create_combiners(table_to_config_dict, table_to_features_dict):
  """Create a per feature list of combiners, ordered by table."""
  combiners = []
  for table in table_to_config_dict:
    combiner = table_to_config_dict[table].combiner or 'sum'
    combiners.extend([combiner] * len(table_to_features_dict[table]))
  return combiners