AMS for a non-AR model, Rails 5.2.3

example.rb

# models/user.rb
class User
  include ActiveModel::Model

  attr_accessor :username, :first_name, :last_name, :id
end

#serializers/user_serializer.rb
class UserSerializer < ActiveModelSerializers::Model
  attributes :id, :first_name, :last_name, :username
end

# controllers/users_controller.rb

class UsersController < ApplicationController
  before_action :init_users
  def index
    render jsonapi: @users
  end

  def show
    @user =  @users.select {|u| u.id == params[:id].to_i}.first
    if @user
      render jsonapi: @user
    else
      render jsonapi: {message: "Wrong user ID"}, status: :not_found
    end
  end

  private

  def init_users
    @users ||= []
    if @users.empty?
      (1...10).to_a.each do |x|
        @users << User.new(
          id: x,
          username: "user-#{x}",
          first_name: "FName-#{x}",
          last_name: "LName-#{x}"
        )
      end
    end

    @users
  end
end

# initializers/ams.rb

# frozen_string_literal: true

# In order to properly handle JSON API responses, we need to register a JSON API renderer,
# like so:
ActiveSupport.on_load(:action_controller) do
  require 'active_model_serializers/register_jsonapi_renderer'
end

ActiveModelSerializers.config.adapter = :json_api
ActiveModelSerializers.config.key_transform = :unaltered