public function delete($id)
 {
     $this->db->trans_start();
     # delete company related by field
     $this->load->model("CompanyModel");
     $company = new CompanyModel();
     $company->delete_by_field($id);
     # remove the field itself
     $condition = array(CompanyFieldModel::$primary_key => $id);
     $this->db->delete(CompanyFieldModel::$table_name, $condition);
     $this->db->trans_complete();
     return $this->db->trans_status();
 }