module Serializer
  def serializables
    self.class.instance_variable_get('@serializables')
  end

  def attr_accessors
    self.class.instance_variable_get('@attr_accessors')
  end

  def self.included(klass)
    klass.send :define_singleton_method, :json, ->(*params) do
      @serializables ||= []
      @serializables.concat params
    end

    klass.send :define_singleton_method, :attr_accessor, ->(*params) do
      @attr_accessors ||= []
      @attr_accessors.concat params
      super(*params)
    end

    klass.send :define_method, :get_instance_variable, ->(key) do
      self.send key.to_sym
    end
  end

  def to_data
    hash = {}
    self.class.instance_variable_get('@serializables').each do |k|
      hash.merge!({"#{k.to_s}" => self.get_instance_variable(k)})
    end
    hash
  end
end

class User
  include Serializer
  attr_accessor :username, :full_name, :password
  json :username, :fullname

end

u = User.new
u.username = "rezam"
u.full_name = "Reza M"
u.password = "secret"

u.attr_accessors
# => [:username, :full_name, :password]
u.serializables
# => [:username, :full_name]
u.to_data
# => {username: 'rezam', full_name: 'Reza M'}