class Profile extends Model {
  static entity = "profiles";

  static fields() {
    return {
      id: this.attr(null),
      user_id: this.attr(null),
      age: this.attr("")
    };
  }
}

class User extends Model {
  static entity = "users";

  static fields() {
    return {
      id: this.attr(null),
      name: this.attr(""),
      profile: this.hasOne(Profile, "user_id")
    };
  }
}

    const user = {
      id: 1,
      name: "Taro",
      profile: {
        id: 2,
        user_id: 1,
        age: 25
      }
    };
    this.before = user.profile;  //  { "id": 2, "user_id": 1, "age": 25, "$id": "2" }
    User.insertOrUpdate({ data: user });
    this.after = user.profile;  //  2