code: def map_db(self, plain_src_db):
          
          if not self._regex_map and not self._plain:
              return [plain_src_db]
          # Lookup this namespace to seed the plain_db dictionary
          self.lookup(plain_src_db + ".$cmd")
          return list(self._plain_db.get(plain_src_db, set()))
docs: Given a plain source database, return the list of target databases.
  
          Individual collections in a database can be mapped to different
          target databases, so map_db can return multiple databases. This
          function must return all target database names so we make the
          following restrictions on wildcards:
          1) A wildcard appearing in the source database name must also appear
          in the target database name, eg "db*.col" => "new_db_*.new_col".
          2) A wildcard appearing in the source collection name must also appear
          in the target collection name, eg "db.col*" => "new_db.new_col*".
  
          This is used by the CommandHelper for the dropDatabase command.