goose-2.2.0.js

var maSchema = new Schema({
    name: { type: String, default: 'nameless' }
  , age: { type: Number, default: function () { return 0 }}
});

var M = mongoose.model('M', maSchema);

// populate some data ...

M.findOne({..}, { age: 1 }, function (err, m) {
  console.log(m.name) // undefined (previously would be "nameless")
  console.log(m.age)  // some value
  console.log(m._id)  // some value
})

M.find({..}, { age: 0 }, function (err, docs) {
  var m = docs[0];
  console.log(m.name) // some value
  console.log(m.age)  // undefined (previously would be 0)
  console.log(m._id)  // some value
})