code: def look_up(self, _from, _localField, _foreignField, _as):
          
          query = {
              'from': _from,
              'localField': _localField,
              'foreignField': _foreignField,
              'as': _as
          }
          self._q.append({'$lookup': query})
          return self
docs: 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