Question

I am using async.js to run a for loop over a collection. For performance reasons, I want to pass in a database connection into the iterator method, so that it’s not opening/closing a db connection each time the iterator runs. I am using mongoose.js for my data models.

The code below gets all the Artists in mongo, and then adds a song for each. My question is, how can I use the same db connection from updateAllArtists in addArtistSong?

function updateAllArtists() {
    var db = mongoose.createConnection('localhost/dbname');
    var Artist = db.model('Artist', artistSchema);
    Artist.find({}, function(err, artists) {
       // for each artist, add a song
       async.forEach(artists, addArtistSong, function(err) {
    });
}

function addArtistSong(artist, cb) {
    // THIS IS WHERE I NEED A DB CONNECTION
    var Song = db.model('Song', songSchema);
}

Can I extend the iterator signature somehow, like addArtistSong(artist, db, cb)? Then how would I pass this in from the forEach call?

Answer 1

One option is to make the db connection a global variable. But if you don’t want that you can always use a closure to create a shared variable between functions:

(function(){
  // create db in this closure
  var db = mongoose.createConnection('localhost/dbname');

  function updateAllArtists() {
    // now db is available here
    var Artist = db.model('Artist', artistSchema);
    Artist.find({}, function(err, artists) {
      // for each artist, add a song
      async.forEach(artists, addArtistSong, function(err) {});
    });
  }

  function addArtistSong(artist, cb) {
    // and also available here
    var Song = db.model('Song', songSchema);
  }
})()

Another option is to pass it as a parameter to addArtistSong. Since async.forEach expect the iterator function to accept only 2 parameters we can use an anonymous function wrapper to pass 3 parameters to addArtistSong:

function addArtistSong(db,artist,callback) {
    db.model(); //...
}

and calling it in async.forEach:

async.forEach(
  artists,
  function(x,cb){addArtistSong(db,x,cb)},
  function(err) {}
);