CODEWITHSUNDEEP
node.jsasynchronouspromiseqnode-mysql
niczak
niczak

Reputation: 3917

Using Q with Node-Mysql

I am relatively new to Node but I have had good success with porting over RESTful API that was previously done in PHP. There is quite a bit of database interaction and as a result I have found myself getting into what I believe many people call "the pyramid of doom" due to the async nature of Node.

As a result I am taking a stab at implementing promises using the Q library but I haven't had much success yet and am still getting null result sets back when I believe I should be getting data. Below is my current structure before adding Q, if anyone could advise on how to correctly implement Q for this I would be able to run with that example and convert the rest of the database/memcached calls over to that model.

// helper function to get a company row
getRow = function(lookUp, callback) {
    var query = db.query('SELECT * FROM table WHERE lookUp = ?', lookUp, function(err, result) {
        var count = Object.keys(result).length;

        if(count == 0) {
            return;
        } else {
            callback(null, result);
        }
    });
}

// function that uses the above helper method
insertItem = function(request, response) {
    var data = JSON.parse(request.body.data);
    var message = data.message;
    var lookUp = data.lookUp;

    security.verifyToken(lookUp, function (lookUpError) {
        if (lookUpError) {
            var errorResult = { "response": "error", "msg": lookUpError };
            response.json(errorResult);
            response.end();
        } else {
            getRow(lookUp, function (companyError, row) {
                var companyId = row[0].id;

                var res = helper.insertFeedItem(companyId, message, function (insertError, insertResult) {
                    var result = (feedError) ? { "response": "error", "msg": insertError} : insertResult;
                    response.json(result);
                    response.end();
                });
            });
        }
    });
}

What I would like to accomplish is being able to do something like:

var result = getCompanyRow(lookUp);
companyId = result.company_id;

Again, any insight into how to best implement Q (or just promises in general) for this case would be extremely appreciated.

* EDIT:

Here is what I have tried thus far to implementing Q, but as I said I am getting nothing back.

function getRow(id) {
  var dfd = Q.defer();
  var query = db.query('SELECT * FROM table WHERE lookUp = ?', id, function(err, result) {
    if(err) { dfd.reject(err); }
    else { dfd.resolve(result); }
  });
  return dfd.promise;
}

The above is not working at all when called as result = getRow(id); I tried using Q.all and binding the function to that but I also got nothing back when trying that approach. I wasn't sure what to include in my call to .then() but I tried a number of things, none were successful.

Upvotes: 3

Views: 4278

Answers (1)

Bergi
Bergi

Reputation: 664936

Your getRow promise function looks promising :-) It can be further simplified by using the node adapter methods from Q:

function getRow(id) {
    return Q.nfcall(db.query,    'SELECT * FROM table WHERE lookUp = ?', id);
    //      ^^^^^^^^^^^^^^^^^^^^
    // or   .ninvoke(db, "query", … if it must be called as a method
}
// or even just
var getRow = Q.nbind(db.query, db, 'SELECT * FROM table WHERE lookUp = ?');

I don't see any benefit to using .then(function ...) because it still requires nesting just like a callback would.

The benefit (apart from easier error handling) comes from chaining multiple tasks, i.e. when also your security.verifyToken and helper.insertFeedItem methods would return promises. If they don't (and you can't modify them) you still can use Q.nfcall as in the example above. Assuming they did, your code can be simplified to

function insertItem(request, response) {
    var data = JSON.parse(request.body.data);

    security.verifyToken(data.lookUp).then(function(/* no lookupError */) {
        return getRow(data.lookUp); // potentially catch SQL errors here
    }).then(function(row) {
        return helper.insertFeedItem(row[0].id, data.message);
        // What was the `res` it had returned before?
    }).catch(function(someError) { // lookUpError, companyError, insertError
        return { "response": "error", "msg": someError };
    }).done(function(result) {
        response.json(result);
        response.end();
    });
}

Upvotes: 4

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