Is an ObjectId automatically generated for a nested object? - mongodb

Question

My schema is as follows:
const MessageType = {
// ...
oAuth: { provider: String, id: String },
attachments: [ {name: String, contentType: String} ],
// ...
}
MessageSchema = new mongoose.Schema(MessageType, { timestamps: true});
Messages = mongoose.model("Message", MessageSchema);
When I insert a new Message document using Messages.create, an ObjectId (_id) is also generated for attachments, in addition to my name and contentType fields, ie:
[ { name: "xxx", contentType: "yyy", _id: zzzzzz }]
Why is this happening, for attachments but not oAuth?

Answer 1

For avoiding that the _id was generated you must set the option _id: false, Also if you don't want to save the empty attachments object, you need to set default: undefined.
const MessageTypeSchema = new mongoose.Schema({
oAuth: {
type: String
},
attachments: {
type: [
{
type: String
}
],
_id: false,
default: undefined
}
});
Here the code that I used to test:
console.log('-------- Document with attachments --------');
new MessageTypeModel({
oAuth:'xxxxxxxxxxxxx',
attachments: ['teste.png','teste2.jpg']
}).save().then(result => {
console.log(result);
});
console.log('-------- Document without attachments --------');
new MessageTypeModel({
oAuth:'xxxxxxxxxxxxx'
}).save().then(result => {
console.log(result);
});
And here the result of execution:
Mongoose creates _id for single nested subdocuments or arrays, and your object field oAuth is not one of this cases:
Subdocuments are documents embedded in other documents. In Mongoose,
this means you can nest schemas in other schemas. Mongoose has two
distinct notions of subdocuments: arrays of subdocuments and single
nested subdocuments.
Each subdocument has an _id by default. Mongoose
document arrays have a special id method for searching a document
array to find a document with a given _id.
var childSchema = new Schema({ name: 'string' });
var parentSchema = new Schema({
// Array of subdocuments
children: [childSchema],
// Single nested subdocuments. Caveat: single nested subdocs only work
// in mongoose >= 4.2.0
child: childSchema
});
Link of Mongoose documentation: Mongoose SubDocs

Answer 2

You can define _id : false in attachments array.
const MessageType = {
// ...
attachments: [ {name: String, contentType: String, _id: false} ],
// ...
}