No _id for single embedded sub documents

Reproducible code:

const mongoose = require('mongoose'),
  Schema = mongoose.Schema;

mongoose.connect('mongodb://localhost/test');

const childSchema = new Schema({
  name: String
});

const parentSchema = new Schema({
  child: childSchema
});

const Parent = mongoose.model('Parent', parentSchema);

mongoose.connection.once('open', function() {
  Parent.create({ child: { name: 'My child' }});
});

Result:

> db.parents.find()
{ "_id" : ObjectId("56eeed1417e215f41643ac8b"), "child" : { "name" : "My child" }, "__v" : 0 }

Expected behaviour is to see a an ObjectId for the child sub document as well. Using the array notation like this works:

const parentSchema = new Schema({
  child: [childSchema]
});

> db.parents.find()
{ "_id" : ObjectId("56eeee3a0110d127174d318e"), "child" : [ { "name" : "My child", "_id" : ObjectId("56eeee3a0110d127174d318f") } ], "__v" : 0 }

Since support for single embedded sub documents has been introduced in 4.2 the first method should work as well unless I’m doing something wrong here.

mongoose version 4.4.4

[vkarpov15]

One man’s bug is another man’s feature 😃

[vkarpov15]

Actually, this was an unintentional bug. Fixed in above, Use { _id: false } in your schema options to avoid getting an _id.