protected function populate(int $requirement_id, &$requirement)
 {
     if ($requirement == null) {
         $requirement = new logistic_requirement((int) $requirement_id);
         $requirement->set_activity_id($this->unmarshal($this->db->f('activity_id'), 'int'));
         $requirement->set_start_date($this->unmarshal($this->db->f('start_date'), 'int'));
         $requirement->set_end_date($this->unmarshal($this->db->f('end_date'), 'int'));
         $requirement->set_no_of_items($this->unmarshal($this->db->f('no_of_elements'), 'int'));
         $requirement->set_location_id($this->unmarshal($this->db->f('location_id'), 'int'));
     }
     return $requirement;
 }