Mongoose.js how to multiply field values during update

I am trying to update the values in a mongodb collection using a mongoose model but i cant seem to make it work the way i want. I would like the value of a particular field to be a multiple of an updated field.

var id = req.user._id

Model.findById({_id: id},function(err,doc){
if(err){

console.log(err)//handle error

)//handle error

}else{
//update file

doc.field1 = req.body.formInput;
doc.field2 = {$multiply: [doc.field1, 2]};
doc.save();
};
});

If anyone can help please, I don't know the right way to do this and its breaking my app. Please help a brother.

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