Node.js : Mongodb - Concurrent Requests - node.js

Question

In my node.js application, a route is executed on Ajax request and it always receives concurrent requests.
My requirement is to check for a similar document and if exist upsert the data to existing document. If no similar document then create a new record.
Refer the code given below. The problem is that most times the "findExistingDoc" returns false, but by the time when a call tries to create a new document, a similar document is already created by a previous call.
Please help with a optimal solution to solve this problem.
self.findExistingDoc(function(err, doc){ // Check if a similar doc already exist
if (!doc){
console.log ("NO existing doc");
self.save(function(err, doc) {
if (err) {
console.error ("DB Error while creating new doc : " + JSON.stringify(err));
} else {
console.log ("New document created");
}
});
} else {
console.log ("existing doc");
var qry = {"name": req.s_name, "class": req.s_class};
self.model("Collection").findOneAndUpdate (qry, {$addToSet: {"data": req.body.s_data}}, {safe: true, upsert: true}, function (err, album) {
if (err) {
console.error ("DB Error while adding data to existing doc : " + JSON.stringify(err));
} else {
console.log ("Data added to existing doc");
}
callback(err, output);
});
}
});

Answer 1

Solved my problem after some googling. Thanks to the new "$setOnInsert" operator that's introduced in mongodb v2.4.
var slug = getUniqueSlug("name");
var qry = {"name": req.s_name, "class": req.s_class};
var update = {"$addToSet": {"data": req.body.s_data}, "$setOnInsert": {"slug": slug}};
self.model("Collection").findAndModify ( qry, update, {"safe": true, "upsert": true} );