nodejs: save function in for loop, async troubles - node.js

Question

NodeJS + Express, MongoDB + Mongoose
I have a JSON feed where each record has a set of "venue" attributes (things like "venue name" "venue location" "venue phone" etc). I want to create a collection of all venues in the feed -- one instance of each venue, no dupes.
I loop through the JSON and test whether the venue exists in my venue collection. If it doesn't, save it.
jsonObj.events.forEach(function(element, index, array){
Venue.findOne({'name': element.vname}, function(err,doc){
if(doc == null){
var instance = new Venue();
instance.name = element.vname;
instance.location = element.location;
instance.phone = element.vphone;
instance.save();
}
}
}
Desired: A list of all venues (no dupes).
Result: Plenty of dupes in the venue collection.
Basically, the loop created a new Venue record for every record in the JSON feed.
I'm learning Node and its async qualities, so I believe the for loop finishes before even the first save() function finishes -- so the if statement is always checking against an empty collection. Console.logging backs this claim up.
I'm not sure how to rework this so that it performs the desired task. I've tried caolan's async module but I can't get it to help. There's a good chance I'm using incorrectly.
Thanks so much for pointing me in the right direction -- I've searched to no avail. If the async module is the right answer, I'd love your help with how to implement it in this specific case.
Thanks again!

Answer 1

Why not go the other way with it? You didn't say what your persistence layer is, but it looks like mongoose or possibly FastLegS. In either case, you can create a Unique Index on your Name field. Then, you can just try to save anything, and handle the error if it's a unique index violation.

Answer 2

Whatever you do, you must do as #Paul suggests and make a unique index in the database. That's the only way to ensure uniqueness.
But the main problem with your code is that in the instance.save() call, you need a callback that triggers the next iteration, otherwise the database will not have had time to save the new record. It's a race condition. You can solve that problem with caolan's forEachSeries function.
Alternatively, you could get an array of records already in the Venue collection that match an item in your JSON object, then filter the matches out of the object, then iteratively add each item left in the filtered JSON object. This will minimize the number of database operations by not trying to create duplicates in the first place.
Venue.find({'name': { $in: jsonObj.events.map(function(event){ return event.vname; }) }}, function (err, docs){
var existingVnames = docs.map(function(doc){ return doc.name; });
var filteredEvents = jsonObj.events.filter(function(event){
return existingVnames.indexOf(event.vname) === -1;
});
filteredEvents.forEach(function(event){
var venue = new Venue();
venue.name = event.vname;
venue.location = event.location;
venue.phone = event.vphone;
venue.save(function (err){
// Optionally, do some logging here, perhaps.
if (err) return console.error('Something went wrong!');
else return console.log('Successfully created new venue %s', venue.name);
});
});
});