code: func (mc MongoClient) AddReading(r contract.Reading) (string, error) {
  	s := mc.getSessionCopy()
  	defer s.Close()
  
  	var mapped models.Reading
  	id, err := mapped.FromContract(r)
  	if err != nil {
  		return "", err
  	}
  
  	mapped.TimestampForAdd()
  
  	err = s.DB(mc.database.Name).C(db.ReadingsCollection).Insert(&mapped)
  	if err != nil {
  		return "", errorMap(err)
  	}
  	return id, err
  }
docs: // Post a new reading