[Ruby] Example of using class attributes

main.rb

require_relative  'user'
require_relative 'user_serializer'

user = User.new('Hello', 'World', '21')

puts UserSerializer.new(user).serialize
# => {:display_name=>"Hello World", :age=>"21 years old"}

serializable_attribute.rb

class SerializableAttribute

  attr_reader :attribute, :block

  def initialize(attribute, &block)
    @attribute = attribute
    @block = block
  end
end

serializer.rb

require 'active_support/concern'
require_relative 'serializable_attribute'

module Serializer
  extend ActiveSupport::Concern

  class_methods do
    attr_accessor :attributes_to_serialize

    def attribute(name, &block)
      self.attributes_to_serialize ||= []
      self.attributes_to_serialize << SerializableAttribute.new(name, &block)
    end
  end

  attr_reader :object

  def initialize(object)
    @object = object
  end

  def serialize
    self.class.attributes_to_serialize.each_with_object({}) do |attribute, result|
      result[attribute.attribute] =
        if attribute.block
          object.instance_eval(&attribute.block)
        else
          object.send(attribute.attribute)
        end
    end
  end
end

user.rb

class User
  attr_reader :last_name, :first_name, :age

  def initialize(first_name, last_name, age)
    @first_name = first_name
    @last_name = last_name
    @age = age
  end

  def display_name
    "#{first_name} #{last_name}"
  end
end

user_serializer.rb

require_relative 'serializer'

class UserSerializer
  include Serializer

  attribute :display_name

  attribute :age do |object|
    "#{object.age} years old"
  end
end