Why new documents in mongo have an object and not an ObjectId?

Question

When inserting new documents in mongodb, ids don't look like ObjectId and instead they look like an object.

"_id" : {
        "_bsontype" : "ObjectID",
        "id" : "U\u0013[-Ф~\u001d$©t",
        "generationTime" : 1.43439e+09
}

Expected type:

"_id" : ObjectId("55107edd8e21f20000fd79a6")

My mongodb version is 3.0.3 and this is pretty much the code and the schema

var Script = {
    run: function() {
        return CourseModel.findQ()
        .then(function(courses){
            return courses.map(worker);
        }).catch(function(error){
            console.log(error);
        });
    }
};

function worker(course){
    var category = { name: course.name, displayOrder: 0 };
        return CategoryModel.createQ(category).then(function() {
            course.set('name', undefined);
            return course.saveQ();
        });
}
module.exports = Script;

var CategorySchema = new Schema({
    name: {
        type: String,
        required: true,
        unique: true
    },
    active: {
        type: Boolean,
        default: true
    },
    displayOrder: Number,
    updateDate: Date,
    subcategories: [{
        type: Schema.Types.ObjectId,
        ref: 'subcategories'
    }]
});

Answer 1

That is what an ObjectID is. It is simply an object that contains those properties.

http://docs.mongodb.org/manual/reference/object-id/

ObjectId is a 12-byte BSON type, constructed using:

• a 4-byte value representing the seconds since the Unix epoch,
• a 3-byte machine identifier,
• a 2-byte process id, and
• a 3-byte counter, starting with a random value.

{
    "_bsontype" : "ObjectID",
    "id" : "U\u0013[-Ф~\u001d$©t",
    "generationTime" : 1.43439e+09
}

U\u0013[-Ф~\u001d$©t is the 12 character binary string which gets converted to the familiar 24 char hex string (55107edd8e21f20000fd79a6) when the object as a whole is represented as a text value (i.e. its .toString function is invoked)

In Mongoose the documents also have a .id getter which give you the 24 char hex as a string value.