i have 2 collections in mongodb User and Review, the schemas are User(_id,firstName,lastName,email,password) and Review(_id, reviewForID,reviewdByID,reviewText), i have an express post method that i will use to return the User and all the reviews associated with that user, but when i try to query the Review collection i sometimes get an empty array returned. the error occurs in the getReviews() function, i have commented the line causeing the error. i do not know what is causeing the error and why it only works when i add toString()

note: the purpose of the getReview() function is to get the array of all the reviews from the db and append that array to the userA object then send the userA object as a response to the client

  var ObjectId = require('mongodb').ObjectID; //used to query objs by id     

 //parameters are (userid)
 app.post('/getUserInfo', function (req, res)
 {
   var queryObj = { _id: new ObjectId(req.body.userid)};

   MongoClient.connect(url, function (err, db)
   {
     if (err)
     {
        console.log('Unable to connect to the mongoDB server. Error:', err);
     }
     else //HURRAY!! We are connected. :)
     {
        var collection = db.collection('User');

        collection.findOne(queryObj, function (err, resultDocument)
        {
           if (err)
           {
              console.log(err);
           }
           else if (resultDocument)
           {
             getReviews(resultDocument,res);
           }
           else
           {
              res.send({"result":"failed"});
           }

           db.close();//Close connection
        });//collection.find end
     }//else
   });//MongoClient.connect end
 });//get user info end

 //find all the reviews and add them to the object userA
 function getReviews(userA,response)
 {


   //var queryObj = { reviewForID: userA._id }; returns empty array
   //var queryObj = { reviewForID: new ObjectId(userA._id) }; returns empty array
   //var queryObj = { reviewForID: userA._id.toString() }; returns correct documents

   MongoClient.connect(url, function (err, db)
   {
      if (err)
      {
         console.log('Unable to connect to the mongoDB server. Error:', err);
      }
      else //HURRAY!! We are connected. :)
      {
          var collection = db.collection('Review');

          collection.find(queryObj).toArray(function (err, result)
          {
             if (err)
             {
                console.log(err);
             }
             else
             {
                response.send(result);
             }

             db.close();//Close connection
         });//coolection.find end

      }//else
  });//MongoClient.connect end
 }//get reviews end