Adds look up stage at query (left outer join)
        :param _from: Specifies the collection in the same database to perform the join with.
        :param _localField: Specifies the field from the documents input to the $lookup stage.
        :param _foreignField: Specifies the field from the documents in the from collection.
        :param _as: Specifies the name of the new array field to add to the input documents.
        :return: The current object

def look_up(self, _from, _localField, _foreignField, _as):
        """
        Adds look up stage at query (left outer join)
        :param _from: Specifies the collection in the same database to perform the join with.
        :param _localField: Specifies the field from the documents input to the $lookup stage.
        :param _foreignField: Specifies the field from the documents in the from collection.
        :param _as: Specifies the name of the new array field to add to the input documents.
        :return: The current object
        """
        query = {
            'from': _from,
            'localField': _localField,
            'foreignField': _foreignField,
            'as': _as
        }
        self._q.append({'$lookup': query})
        return self