How to limit adding a document to MongoDB based on the result of a check?

Question

MongoDB

Now the existing construction for some reason adds the document to the database based on the result of the check, in any case, the entry to the database is triggered even if false.

>> Handler

chatCreate = (req, res) => {

    const chat = new Chat({
        nickname: req.body.nickname,
        message: req.body.message
    });

    chat.save()
        .then(chat => {

            Room.find({
                'room_name': {$in: req.body.room}
            }, (err, rooms) => {

                if (err) {
                    return res.send(err);
                }

                 // тут проверяем наличие записи в БД и по результату выполняем действие
                if (rooms.length !== 0) {

                  chat.room = rooms;
                  chat.save(function (err) {

                      if (err) {
                          return res.send(err);
                      }

                      let answer   = {};
                      answer.state = 'success';
                      answer.text  = 'Message post successfully!';
                      let newarray = answer.concat(chat);
                      res.send(newarray);
                  });
                } else {
                   return res.send(err);
                }
            });
        })
        .catch(err => {
            res.status(500).send(err);
        });
};

>> Room storage base model

const mongoose = require('mongoose');
const RoomSchema = mongoose.Schema({

    room_name: String,
    created_date: { type: Date, default: Date.now }
});

module.exports = mongoose.model('Room', RoomSchema);

>> Message storage database model

const mongoose = require('mongoose'), Schema = mongoose.Schema;
const ChatSchema = mongoose.Schema({

    room: [{ type: Schema.Types.ObjectId, ref: 'Room' }],
    nickname: String,
    message: String,
    created_date: { type: Date, default: Date.now }
});

module.exports = mongoose.model('Chat', ChatSchema);

>> Even if the condition if (rooms.length !== 0) > false , the document is still added to the database