Mongoose populate() returns undefined - node.js

Question

I am trying to create a chat with node.js, Express & MongoDB. I have two mongoose models: for chat room and message.
Room model:
const RoomSchema = new mongoose.Schema({
name: {
type: String,
required: true,
},
owner: {
type: mongoose.Schema.Types.ObjectId,
ref: 'Users',
required: true,
},
messages: {
type: [mongoose.Schema.Types.ObjectId],
ref: 'Messages',
required: true,
},
users: {
type: [mongoose.Schema.Types.ObjectId],
ref: 'Users',
required: true,
},
});
const Room = mongoose.model('Rooms', RoomSchema);
Message model:
const MessageSchema = new mongoose.Schema({
text: {
type: String,
required: true,
},
sendBy: {
type: String,
required: true
}
});
const Messages = mongoose.model('Messages', MessageSchema);
and a function for displaying all messages
prepareMessages: function (name, callback) {
rooms.findOne({
name: name,
}).populate('messages')
.exec(function (err, room) {
let result = '';
if (!room) {
console.log("Chat does not exist");
throw err;
}
else {
room.messages.forEach(function (item, i, arr) {
result += '<dt>' + item.sendBy + '</dt>';
result += '<dd>' + item.text + '</dd>';
});
}
callback(result)
});
},
Why do I get
TypeError: Cannot read property 'sendBy' of undefined?
I tried .populate('rooms.messages'), .populate({path: 'rooms.messages', model:'Messages') and another variants, but it still doesn't work. Help me, please!

Answer 1

In the Room Schema, you're defining the messages as an object with type of Array of ObjectId, while it should be an array of elements, each element is of type ObjectId, messages should be an array of ObjectIds, and users as well
const RoomSchema = new mongoose.Schema({
name: {
type: String,
required: true,
},
owner: {
type: mongoose.Schema.Types.ObjectId,
ref: 'Users',
required: true,
},
messages: [{ // note here messages is an array
type: mongoose.Schema.Types.ObjectId,
ref: 'Messages',
required: true,
}],
users: [{
type: mongoose.Schema.Types.ObjectId,
ref: 'Users',
required: true,
}],
});
const Room = mongoose.model('Rooms', RoomSchema);
hope it helps

Answer 2

sendBy should be of type ObjectId
const MessageSchema = new mongoose.Schema({
text: {
type: String,
required: true,
},
sendBy: {
type: mongoose.Schema.Types.ObjectId,
required: true
}
});
const Messages = mongoose.model('Messages', MessageSchema);
ref should be in lowercase.
ref: 'Users', should be changed to ref:'users'
Similarly at all other refs