Set Mongoose model instance properties with "extend" util
What you can do is to use Mongoose's update method:
Song.update({ song_id: '123',}, { $set: { title: 'changed title', artist: { name: 'changed artist', id: '789', }, },}, function(err, numAffected) { ...});However, I don't actually see why your extend attempt failed. It seems to work for me, I usually use underscore's _.extend function.