Sails.js best practice in using transactions with promises (Postgres)

Question

I\'m using sails 0.9.16 with Postgres and my question is: what is the best way to execute transaction using current API with promises? May be there is something better than:

Answer 1

I'm currently using this exact workflow. For executing one query with promises do this:

Model
 .query(params)
 .then(function(result){
 //act on result
 })
 .catch(function(error){
 //handle error
 })
 .done(function(){
 //clean up
 });

To execute multiple queries in parallel, do this:

var Promise = require('q');

Promise.all([

    User.findOne(),
    AnotherModel.findOne(),
    AnotherModel2.find()

])
.spread(function(user,anotherModel,anotherModel2){
    //use the results
})
.catch(function(){
    //handle errors
})
.done(function(){
    //clean up
});

If you're trying to avoid nesting in your code:

Model
.query(params)
.then(function(result){//after query #1
    //since you're returning a promise here, you can use .then after this
    return Model.query();
})
.then(function(results){//after query#2
    if(!results){
        throw new Error("No results found in query #2");
    }else{
        return Model.differentQuery(results);
    }

})
.then(function(results){
//do something with the results
})
.catch(function(err){
    console.log(err);
})
.done(function(){
    //cleanup
});

Note: currently, waterline uses Q for promises. There is a pull request to switch waterline from Q to bluebird here: waterline/bluebird

When I answered this question, I'd yet to take the database class in college, so I didn't know what a transaction was. I did some digging, and bluebird allows you to do transactions with promises. The only problem is, this isn't exactly built into sails since it's some what of a special use case. Here's the code bluebird provides for this situation.

var pg = require('pg');
var Promise = require('bluebird');
Promise.promisifyAll(pg);

function getTransaction(connectionString) {
    var close;
    return pg.connectAsync(connectionString).spread(function(client, done) {
        close = done;
        return client.queryAsync('BEGIN').then(function () {
            return client;
        });
    }).disposer(function(client, promise) {
        if (promise.isFulfilled()) {
            return client.queryAsync('COMMIT').then(closeClient);
        } else {
            return client.queryAsync('ROLLBACK').then(closeClient);
        }
        function closeClient() {
            if (close) close(client);
        }
    });
}

exports.getTransaction = getTransaction;

Answer 2

The best way to deal with transactions is when they are wrapped properly by a promise library, because transaction logic maps perfectly into the promise event chain, one doesn't have to worry about when to do COMMIT or ROLLBACK, as it happens automatically.

Here's a complete example of how it works with pg-promise library:

var pgp = require('pg-promise')(/*options*/);

var cn = "postgres://username:password@host:port/database";
var db = pgp(cn); // database instance;

db.tx(t => {
    // BEGIN has been executed
    return t.batch([
        t.one("insert into users(name) values($1) returning id", 'John'),
        t.one("insert into users(name) values($1) returning id", 'Mike')
    ]);
})
    .then(data => {
        // COMMIT has been executed
        console.log(data[0].id); // print id assigned to John;
        console.log(data[1].id); // print id assigned to Mike;
    })
    .catch(error => {
        // ROLLBACK has been executed
        console.log(error); // print why failed;
    });