Im using ExpressJS and MongooseJS:
exports.show = function(req, res) {
    var id = req.params.id;
    Subject.findById(id, function(err, subject) {
        if ( err ) {
            return res.json(err);
        }
        res.render('subject/view', {subject: subject});
    });
};
When id is invalid, users got:
{
  "message": "Cast to ObjectId failed for value \"some_id\" at path \"_id\"",
  "name": "CastError",
  "type": "ObjectId",
  "value": "some_id",
  "path": "_id"
}
How to check id is valid before passing it to findById?
Thanks
