vuex-orm: Why vuex-orm returns null in relationship field?

I followed the guide of Defining Relationships in Vuex ORM.

I did everything like the guide, so why I get null value in the category field in Article Model?

codesandbox

This is how I defining my models (category on article is: hasOne/belongTo):

export class Category extends Model {
  static entity = "categories";

  static primaryKey = "_id";

  static fields() {
    return {
      _id: this.attr(null),
      name: this.attr("")
    };
  }
}


export class Article extends Model {
  static entity = "articles";

  static primaryKey = "_id";

  static fields() {
    return {
      _id: this.attr(null),
      name: this.attr(""),
      category: this.hasOne(Category, "_id")
    };
  }

Config Vuex-orm with vuex in main.js:

import VuexORM from "@vuex-orm/core";
import { Article, Category } from "./models";

Vue.use(Vuex);

const database = new VuexORM.Database();

const Articles = {
  namespaced: true,

  actions: {
    test() {
      console.log(this);
    }
  }
};

const Categories = {
  namespaced: true,

  actions: {
    test() {
      console.log(this);
    }
  }
};

database.register(Article, Articles);
database.register(Category, Categories);

const store = new Vuex.Store({
  plugins: [VuexORM.install(database)]
});

Inside app component I have the data that I insert to vuex and get the values like so:

 const articleData = [
      {
        _id: "6ce9bae00000000000000000",
        name: "article-1",
        category: "5ce9acd00000000000000000"
      }
    ];
    const categoryData = [
      { _id: "5ce9acd00000000000000000", name: "category-1" }
    ];

    const x = await Article.insertOrUpdate({ data: articleData });
    const y = await Category.insertOrUpdate({ data: categoryData });

    const a = Article.query()
      .with("category")
      .first();

    console.log({ a });
    console.log({ type: a.category });
    console.log("why a.category is null???");

I have been trying to understand this for several days without success. I would much appreciate any help.