Chai test a mongoose Promise - express

Question

I am really struggling with Chai testing especially now that I have started using mongoose Promises and Async/Await.
Apologies in advance for the long post, but each part is necessary to form a bigger picture.
This is a Message Board app (FCC for anyone familiar).
3 models for Board, Thread, Reply with Ref.
My Thread Schema needs a Board _id when created.
In my Chai test I am hardcoding a message Board _id. This Thread Object does get created in the database but the board field is not added and I can't understand why.
The second issue is that the CHAI test does not return a body in the response - it returns {}. So I have nothing to run asserts against. The newThread function does return a data object from the new_thread.save(function (err, data), as I had the new _id to the Board/threads sub-document array
Thread Schema
const ThreadSchema = new Schema({
"text": {'type': String},
"delete_password": { 'type': String,'select': false},
"created_on": {'type': Date, 'default': new Date()},
"bumped_on": {'type': Date, 'default': new Date()},
"reported": {'type': Boolean,'select': false,'default': false},
"replycount":{'type': Number,'default': 0},
"board": {'type': mongoose.Schema.Types.ObjectId,'ref': 'Board'},
"replies": [{'type': mongoose.Schema.Types.ObjectId,'ref': 'Reply'}]
});
Create a Thread function
this.newThread = function(req, res) {
let threadText = req.body.text;
let passwordText = req.body.delete_password;
let boardTitle = req.params.board;
const new_thread = new models.Thread({
text: threadText,
delete_password: passwordText,
board:currentBoardId //currentBoardId defined & set earlier
});
new_thread.save(function (err, data) {
models.Board
.findOne({ board_title: boardTitle})
.then(board => {
if (board == null) {
res.json({ message: "Cannot find board named, " + boardTitle });
} else {
board.threads.push(data._id);
board.save(function (err){
if (err) {console.log(err);}
res.end();
});
}
})
.catch(err => {
return res.status(500).json({ message: err.message })
})
})
};
CHAI test
test('Every field filled in', function(done) {
chai.request(server)
.post('/api/threads/test')
.send({
board: ObjectId('5d8f748a1d788a3be2b9a7b7'), // board test _id Never gets added to database
text: 'POST - new thread - test thread text',
delete_password: 'password'
})
.end(function(err, res){
expect(err).to.be.null;
assert.equal(res.status, 200);
console.log(res.body); // returns {}
done();
});
});