code: function destroy(instance, tx, callback) {
      var
          model = instance.__model,
          sql = utils.format('delete from `%s` where `%s`=?', model.__table, model.__primaryKey),
          params = [instance[model.__primaryKey]];
      smartRunSQL(model.__pool, sql, params, tx, function (err, result) {
          if (err) {
              return callback(err);
          }
          callback(null, instance);
      });
  }
docs: delete an instance by id: