node.js - async waterfall callback isn't called within mongoose save -

i'm using express, mongoose, , async.

within update method on controller, i'm calling following:

//note: we're within constructor's prototype method, hence 'self'. body request body.      async.waterfall([         function(callback) {             self.collection.findbyid(body._id).exec(function(err, item) {                 if(item) {                     callback(null, item);                 } else {                     callback(err);                 }             });         },         function(item, callback) {             //callback(null, item); //makes task work, not need              //merge 2 models together, save             item.save( _.extend(item, body), function(err, item) {                 console.log('code here never seems called within waterfall');                 callback(null, item);             });         }     ], function(err, results) {         console.log('hello', err, results);         self.res.json(results);     }); 

so i'm trying find document id, merge new object in 1 found, save it, , return result json. callback function nested within .save never seems called. entire request seems hang , final function in waterfall never gets called.

i might wrong, seems save method's callback gets called asynchronously. how go getting work?

side note: if save method's callback async, why seem work?

    var _this = this;     var item = new this.collection(this.req.body);     item.save(function(err, data) {         _this.res.json(data);     }); 

that method returns saved object json fine.

you not using model.save method correctly. takes callback first argument. must fetch item instance, set new property values on item instance, item.save(callback);.