code: func (mc MongoClient) IntervalById(id string) (contract.Interval, error) {
  	s := mc.getSessionCopy()
  	defer s.Close()
  
  	query, err := idToBsonM(id)
  	if err != nil {
  		return contract.Interval{}, err
  	}
  
  	var interval models.Interval
  	if err := s.DB(mc.database.Name).C(db.Interval).Find(query).One(&interval); err != nil {
  		return contract.Interval{}, errorMap(err)
  	}
  	return interval.ToContract(), nil
  }
docs: // Return an Interval by ID
  // UnexpectedError - failed to retrieve interval from the database