'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.'

def map_db(self, plain_src_db):

    if ((not self._regex_map) and (not self._plain)):
        return [plain_src_db]
    self.lookup((plain_src_db + '.$cmd'))
    return list(self._plain_db.get(plain_src_db, set()))
