Кирилл

let s = new Subscriber({name:'Sn'});
let p = new Post({title:'Pt',subscriber:s });
let u = new User({name:'Un', post: p});

s.save();
p.save();
u.save();

[
  {
    "_id": "58a81fd04396bb59443950e0",
    "name": "Un",
    "post": {
      "_id": "58a81fd04396bb59443950df",
      "title": "Pt",
      "subscriber": {
        "_id": "58a81fd04396bb59443950de",
        "name": "Sn",
        "__v": 0
      },
      "__v": 0
    },
    "__v": 0
  }
]