code: def _as_document(self, partition):
          
          doc = super(self.__class__, self)._as_document(partition)
  
          # pass time_coverage to the _index_document.
          doc['time_coverage'] = partition.time_coverage
          return doc
docs: Converts partition to document indexed by to FTS index.
  
          Args:
              partition (orm.Partition): partition to convert.
  
          Returns:
              dict with structure matches to BasePartitionIndex._schema.