docs: def _as_document(self, partition):
          """ Converts partition to document indexed by to FTS index.
  
          Args:
              partition (orm.Partition): partition to convert.
  
          Returns:
              dict with structure matches to BasePartitionIndex._schema.
  
          """
          doc = super(self.__class__, self)._as_document(partition)
  
          # SQLite FTS can't find terms with `-`, replace it with underscore here and while searching.
          # See http://stackoverflow.com/questions/3865733/how-do-i-escape-the-character-in-sqlite-fts3-queries
          doc['keywords'] = doc['keywords'].replace('-', '_')
          doc['doc'] = doc['doc'].replace('-', '_')
          doc['title'] = doc['title'].replace('-', '_')
  
          # pass time_coverage to the _index_document.
          doc['time_coverage'] = partition.time_coverage
          return doc