.get(function(req,res){
      //returning aggregate values from comments part.. comment.authorId must hv user.username.
       CardComment.aggregate([ //stackoverflow qn... ask tomorrow  show the models
           //now perform the mongo join  of multiple collections like in SQL Joins
           {
             $lookup:{
                from: 'users', //external collection.
                localField: 'authorId', //input field., from current collection.. cardComments
                foreignField: '_id', //foreign key from external collection,
                as: 'commentUser'
             }
           },
           //filter according to the cardId.. part. find()... first pipeline
           {
             $match:{"card": req.params.cardId }
           }

       ], function(err,comments){
         if (err) {
            res.json({"success":false, "message": 'Error in loading comments'})
         } else {
             //res.json({"success": true, "message": comments})
              console.log(JSON.stringify(comments))
            //console.log(comments) //u need to ask for help on stackoverflow.
         }
       })

    })

authorId:{
      type: ObjectId,
      ref : 'User',
      required: true
    },
    createdAt: {
       type: Date,
       default: Date.now
    }

var UserSchema = new Schema({
username: { type: String, required: true, lowercase: true, index: { unique: true } },
email: { type: String, required: true, index: { unique: true } },
//password: { type: String, required: true }  u forgot the select field.. that z why login was disturbing
 password: {type: String, select: false} //that z why u need postman to test stuff