def add_fields(self, **fields):
query = {}<EOL>for field in fields:<EOL><INDENT>query[field] = fields[field]<EOL><DEDENT>self._q.append({'<STR_LIT>': query})<EOL>return self<EOL>
Adds a stage to aggregation query which adds new fields to documents.
:param fields: The fields that will be created
:return: The current object