Wednesday, March 23, 2016

An Express middleware to check if a resource exists

After writing a similar code snippets for tens of times, I decided to write a middleware to handle it. The scenario is quite common. You want to check if an resource exists before going to the next step of processing. In my applications,  it is a query in the MongoDB.


/**
 * A middleware to check if id exists in collection
 * app.get('/resources/:id/', exist('id', collection), function(req, res){...}}
 * @param  {String} id            the parameter name of item id in req object
 * @param  {Model} collection     the collection model
 * @return {Function}             the middleware
 */
function exist(id, collection) {
  return function (req, res, next) {
    collection.findById(req.params[id]).exec(function (err, item) {
      if (err) {
        console.error(err);
        return res.send(500, 'Something wrong!');
      }

      if (!item) {
        return res.send(404, 'item ' + req.params[id] + ' not found');
      }
      
      req[req.params[id]] = item;
      next();
    });
  };
}