Select fields bringing more fields than expected

[David Rissato Cruz]

I don't know if this is a mongoose or a mongo issue, but running this example:

var mongoose = require('mongoose');

mongoose.connect("mongodb://localhost:27017/test");

var sampleSchema = new mongoose.Schema({
    _id: Number,
    name: String,
    approval: {
        approved: {type: Boolean, default: false},
        approver: String
    }
});

var Sample = mongoose.model("Sample", sampleSchema);


Promise
    .all([
        (new Sample({_id: 1, name: "test1"})).save(),
        (new Sample({_id: 2, name: "test2"})).save(),
        (new Sample({_id: 3, name: "test3", approval: {approver: "David"}})).save()
    ])
    .then(function () {
        return Sample.find().select("_id").exec(); // ** WE WANT _id FIELD ONLY
    })
    .then(console.log)
    .catch(console.error)
    .then(mongoose.disconnect.bind(mongoose));

Returns an array like that:

[ { _id: 1, approval: {} },
  { _id: 2, approval: {} },
  { _id: 3, approval: {} } ]

Where I expected that:

[ { _id: 1},
  { _id: 2},
  { _id: 3} ]

Does anybody know why mongoose behave like that? Isn't it a bug?