Trying to delete/set null a field with findOneAndUpdate

I’m trying to delete (also setting it to null would be acceptable as a backup plan) the field “board” from a document that looks like this:

var cardSchema = new Schema({
  title: {
    type: String,
    required: true
  },
version: {
    type: Number,
    required: true,
    "default": 0
  },
  board: {
    type: ObjectId,
    ref: 'Board'
  },
},{strict: false});
mongoose.model('Card', cardSchema);

For my scenario, I need to use the function findOneAndUpdate in the following way:

var cardChanges = {board: undefined, version: 10};
Card
  .findOneAndUpdate({_id: "my_id"}, cardChanges, 
      {"new": true, upsert: false, passRawResult: false, 
        "overwrite": false, runValidators: true, 
         setDefaultsOnInsert: true})
  .exec(function(err, result) {
    if(err) {
      // error handlers
    }
    if(!result) {
      // No card to be updated
    }
    // ok
  });

Before the update I have the document:

{
_id: "my_id",
title: "This is my card",
board: "my_board",
version: 9
}

And after:

{
_id: "my_id",
title: "This is my card",
board: "my_board",
version: 10
}

I tried to set the board with both null and undefined, the result is the same. For my use case I can’t use the save method. Mongoose 4.5.5, MongoDB 3.2.9 Any idea or workaround?

Thanks

[vkarpov15]

var cardChanges = { $set: { version: 10 }, $unset: { board: 1 } } try that as a workaround

[vkarpov15]

Happy to help 👍