How to create a Mongoose schema with an array and references to it in the same document?

Question

MongoDB

There is a conditional model of the following content:

...
attachments: [{
  _id: true,
  alt: { type: String, default: '' },
  url: { String }
}],
content: [{
  title: String,
  text: String,
  attachments: [
    // тут нужно ссылатся на this.attachments._id
  ]
}]
...

Please tell me how to set up links from "content.attachments" to the "attachments" array and how to do populate() with this approach?