I get error when I try to save in mongoose?

I am trying to save some data in discussionReplyingSchema and than connect it to the discussion Schema. Those are my two schematics for discussions and replies:

const mongoose = require('mongoose');
const ObjectID = mongoose.Schema.Types.ObjectId;

let discussionSchema = mongoose.Schema({
    title: {type: String, required: true},
    content: {type: String},
    author: {type: ObjectID, required: true, ref: 'User'},
    date: {type: Date, default: Date.now()},
    reply: [{ type: ObjectID, ref: 'Replying' }]
});

let discussionReplyingSchema = mongoose.Schema({
    content: {type: String, required: true},
    author: {type: mongoose.Schema.Types.ObjectId, required: true, ref: 'User'},
    date: {type: Date, default: Date.now()}
});

const Discussion = mongoose.model('Discussion', discussionSchema);
const Replying = mongoose.model('Replying', discussionReplyingSchema);

module.exports = Discussion;
module.exports = Replying;

In the controller I get the user and content and trying to save them in mongoose but I get error. Code:

replyPost: (req, res) => {
        let replyParts = req.body;

        let replyContent = req.body.replyContent;
        console.log(replyContent);

        let userId = req.user.id;
        replyParts.author = userId;

        Replying.create(replyParts).then(reply => {
            req.user.reply.push(reply.id);
            req.user.save(err => {
               if (err) {
                res.render('/');
               } else{
                res.redirect('/');
               }
            });
        });
    }

That's how the database is right now:

"reply": [],

I am doing it for my project so if possible can someone explain to me where is the bug and fix it if possible?

via shefasf