How to keep values from reverting back to default during update in mongoose

I have a mongo document with some default behavior

var thingSchema = new mongoose.Schema(
    things:
        first: {
            type: Boolean,
            default: false
        },
        second: {
            type: Boolean,
            default: false
        }
); 

var myThings = mongoose.model('Things', thingsSchema);

If I then update first

myThings.update({things: {first: true}});

and output Things

Things.find(function(err, things){
     console.log(things) // {things: {first: true}, {second: false}}
});

and I then update the second value

myThings.update({things: {second: true}});

I get {things: {first: false}, {second: true}} when I output again, instead of the desired {things: {first: true}, {second: true}}.

Without having to pass both first and second values to update each time, how can I keep my model from reverting back to its default behavior?