Insert an ObjectId to database?

I still can’t figure this out…
my mongoose model as a ref field:

const ThreadSchema = new Schema({
 "board" : {
    'type': mongoose.Schema.Types.ObjectId,
    'ref': 'Board'

In a chai test, when I try to insert an object … it inserts all fields as expected EXCEPT the ‘board’ field. Does anyone know why?

           boardTitle: 'test',           
           text:'POST - new thread - thread text', 
           board: '5d9a41be671eb600c4609782' // also tried: ObjectId('5d9a41be671eb600c4609782')