Delete a key from a MongoDB document using Mongoose

In early versions, you would have needed to drop down the node-mongodb-native driver. Each model has a collection object that contains all the methods that node-mongodb-native offers. So you can do the action in question by this:

User.collection.update({_id: user._id}, {$unset: {field: 1 }});

Since version 2.0 you can do:

User.update({_id: user._id}, {$unset: {field: 1 }}, callback);

And since version 2.4, if you have an instance of a model already you can do:

doc.field = undefined;
doc.save(callback);

You'll want to do this:

User.findOne({}, function(err, user){
  user.key_to_delete = undefined;
  user.save();
});