node.js + sequelize + sequential execution

Question

In my node application i have to execute 2 queries 1 after the other (i.e) I have to execute 2nd query based on result of 1st query.

My code:

 var levels;
        try {
            sequelize.query("Select * from levels where country_id = " + level0 + "").success(function(results) {
                levels = results;               
            }).failure(function(err) {
                if (err) {
                    logger.error(err.stack);
                    throw (err);
                }
                else {
                    if (callback) {
                        callback(err, null);
                    }
                }
            });
        }
        catch (err) {
        }


if (level == 2) {
        query = "select *from xxxxxx where " + query + " and leveltype = "+levels[0].name_2+"";-----------------------------------------------------------> this is from 1st query

        try {
            sequelize.query(query).success(function(results) {
                results = tsv.stringify(results);
                onSuccess(results, callback)
            }).failure(function(err) {
                if (err) {
                    logger.error(err.stack);
                    throw (err);
                }
                else {
                    if (callback) {
                        callback(err, null);
                    }
                }
            });
        }
        catch (err) {
            logger.error(err.stack)
        }
    }

Please help me to solve this. Thanks in advance.

Answer 1

Sequelize uses Bluebird promises:

sequelize.query("Select * from levels where country_id = " + level0 + "").success(function(results) {
                levels = results;               
            }).failure(function(err) {
                if (err) {
                    logger.error(err.stack);
                    throw (err);
                }
                else {
                    if (callback) {
                        callback(err, null);
                    }
                }
            });
        }

Can be turned into:

sequelize.query("Select * from levels where country_id = " + level0 + "")
         .then(function(results)  {
            levels = results;               
        });

Promises chain:

sequelize.query("Select * from levels where country_id = " + level0 + "")
.then(function(levels)  {
     var query = "select *from xxxxxx where " + query + 
                 " and leveltype = "+levels[0].name_2+""
     return sequelize.query(query);
 }).nodeify(callback); // turn back from promise API to callback API

And this code basically does everything your above code does :)

• You don't need try/catch and if(err) checks with promises - they take care of it for you. If you throw inside a promise handler you're safe.
• Promises chain, if you return the value of a promise from a .then it'll resolve it with that promise (meaning you can add a .then handler)

You can do better though, you can return the value of the promise (that is - return sequelize.query) and switch to a promise API entirely.