code: func (mc MongoClient) getValueDescriptor(q bson.M) (models.ValueDescriptor, error) {
  	s := mc.getSessionCopy()
  	defer s.Close()
  
  	var m models.ValueDescriptor
  	if err := s.DB(mc.database.Name).C(db.ValueDescriptorCollection).Find(q).One(&m); err != nil {
  		return models.ValueDescriptor{}, errorMap(err)
  	}
  	return m, nil
  }
docs: // Get a value descriptor based on the query