Why is the whole "scheme" littered when an element is added to a user?

Question

MongoDB

Hello!
The functionality adds a new post to a specific user's array. Here is the code:

Router.route("/newpost").post(function(req, res) {
  UserSchema.findById(req.body.user).then((user) => {
    const id = function() {
      return (
        "_" +
        Math.random()
          .toString(36)
          .substr(2, 9)
      );
    };
    req.body.data.key = id();
    user.posts.unshift(req.body.data);
    user.save();
    console.log("--- user", user);
    res.send(user);
  });
});

Schema :

const mongoose = require("mongoose")
const Schema = mongoose.Schema

const UserSchema = new Schema({
    name : {
        type: String
    },
    login : {
        type: String
    },
    posts : {
        type : Array
    },
    email : {
        type: String
    },
    password : {
        type: String
    },
})

module.exports = mongoose.model("UserSchema", UserSchema, "users")

Adds successfully, but also creates an empty object in the "users" directory:

What causes this behavior? I want it to only add to the con th user's array.
Thank you!