Presently there is no way to refer to another field value within any form of update
statement in MongoDB in general, so you are generally stuck with "finding" the item and then issuing a separate .save()
method:
Model.findById(id_array[i],function(err,found) {
found.value = ((doc.score)^3)/2;
found.save();
});
Additional callback in the save is optional ( advised ) to your purpose, but this is the only current way to set one field based on the value of another. So bulk updates are right out unless you are looping.