Add query part to ``search``.

        Override this if you wish to customize the query used.

def query(self, search, query):
        """
        Add query part to ``search``.

        Override this if you wish to customize the query used.
        """
        if query:
            if self.fields:
                return search.query('multi_match', fields=self.fields, query=query)
            else:
                return search.query('multi_match', query=query)
        return search