I am new to node.js so this might be a silly mistake. I have a mongodb schema like this:

seller_schema = mongoose.Schema({    
    name:String,
    email:String,
    trainers:[{ type: mongoose.Schema.Types.ObjectId, ref: 'trainers' }]
});
seller_collection=db.model('seller',seller_schema);

and trainer_collection is :

trainer_schema = mongoose.Schema({
        trainer_fname:String,
        trainer_lname:String,
        trainer_photo:String
        });
trainer_collection=db.model('trainers',trainer_schema);

I have a function that adds seller to database. Number of trainers vary upon situation. So, I am implementing a $push to add additional trainers before calling .save function.

function createSeller(req, res){
var seller = new seller_collection({
        name: req.query.first_name,
         email:req.query.email

});
for(i=0;i<req.query.num_trainers;i++){
    try{
    var trainer_object={ "trainer_fname": req.query.trainer_fname,
                         "trainer_lname": req.query.trainer_lname,
                         "trainer_photo": req.query.trainer_photo
                         }
    console.log(seller._id);
        seller_collection.update({ _id: seller._id },{
                $push: {
                    "trainers":trainer_object
                }
        },function(error,course){
            console.log(error);
            if(error){
                res.status(500).json({
                    message: "failure",
                    status_code:"500",
                    error:error
                });
            }else{
                res.status(200).json({
                    message: "success",
                    status_code:"200"
                });
            }
        });

     }catch(ex){
        console.log(ex);
        res.status(500).json({
                    message: "failure",
                    status_code:"500",
                    res:res
        });
     }
}
}

Stacktrace:

{ [CastError: Cast to undefined failed for value "[object Object]" at path "trainers"]
      stack: 'Error\n    at MongooseError.CastError (C:\\path\\node_modules\\mongoose\\lib\\error\\cast.js:18:16)  at
    SchemaArray.cast (C:\\path\\node_modules\\mongoose\\lib\\schema\\array.js:156:15) at SchemaArray.cast (C:\\path\\node_modules\\mongoose\\lib\\schema\\array.js:167:17) at Query._castUpdateVal (C:\\path\\node_modules\\mongoose\\lib\\query.js:2357:22)\n    at Query._
    walkUpdatePath (C:\\path\\node_modules\\mongoose\\lib\\query.js:2273:27) at Query._castUpdate (C:\\path\\node_modules\\mongoose\\lib\\query.js:2202:23)
     at Query.update (C:\\path\\node_modules\\mongoose\\lib\\query.js:2035:22)\n    at Function.update (C:\\path\\node_modules\\mongoose\\lib\\model.js:174
    1:13) at createSeller (C:\path\\routes\\seller.js:269:31)  at Query.<anonymous> (C:\\path\\routes\\seller.js:12:21)  at C:\\path\\node_modules\\mongoose\\node_modules\\kareem\\index.js:177:19  at C:\\path\\node_modules\\mongoose\\node_modules\\kareem\\index.js:109:16 at process._tickCallback (node.js:355:11)',
      message: 'Cast to undefined failed for value "[object Object]" at path "trainers"',
      name: 'CastError',
      kind: undefined,
      value: [{"trainer_photo":"x"}],
      path: 'trainers' }

For anyone out there facing same problem.

I changed trainers field in schema like this trainers:[trainer_schema] and edited $push method to seller.trainers.push(trainer_object);