/**
  * Adds a field to select fields.
  * 
  * You can either use this individual function or use setQuery to set the whole
  * query object.
  * 
  * @param string $field The value of the field.
  * 
  * @return none.
  */
 public function addSelectField($field)
 {
     $this->_query->addSelectField($field);
 }