Rails PORO Decorator - Supports ActiveAdmin

application_presenter.rb

class ApplicationPresenter
  attr_reader :model

  delegate_missing_to :@model
  delegate :helpers, to: 'ActionController::Base'
  delegate :url_helpers, to: 'Rails.application.routes'
  delegate :to_param, :to_json, :to_query, :to_yaml, :to_enum, to: :@model
  delegate :model_name, :model_name_const, :model_name_underscore, to: :class

  def initialize(model)
    @model = model
    instance_variable_set("@#{model_name_underscore}", model)

    class_eval do
      attr_reader model_name_underscore
    end
  end

  def decorated?
    true
  end

  class << self
    delegate :helpers, to: 'ActionController::Base'
    delegate :all, :arel_table, :find_by_sql, :columns, :connection,\
             :unscoped, :table_name, :primary_key, to: :model_name_const

    def model_name_const
      name = self.name || superclass.name
      name.gsub(/Presenter\z/, '').constantize
    end

    def model_name_underscore
      model_name_const.to_s.underscore
    end

    def model_name
      ActiveModel::Name.new model_name_const.to_s.constantize
    end

    def build_default_scope
      model_name_const.send(:build_default_scope)
    end

    def decorate(*args)
      collection_or_object = args[0]
      if collection_or_object.respond_to?(:to_ary)
        DecoratedEnumerableProxy.new(collection_or_object, model_name_const)
      else
        new(collection_or_object)
      end
    end
  end

  class DecoratedEnumerableProxy < DelegateClass(ActiveRecord::Relation)
    include Enumerable

    delegate :as_json, :collect, :map, :each, :[], :all?, :include?, \
             :first, :last, :shift, to: :decorated_collection

    def initialize(collection, class_name)
      super(collection)
      @class_name = class_name
    end

    def klass
      "#{@class_name}Presenter".constantize
    end

    def wrapped_collection
      __getobj__
    end

    def decorated_collection
      @decorated_collection ||= wrapped_collection.collect { |member| klass.decorate(member) }
    end
    alias to_ary decorated_collection

    def each(&blk)
      to_ary.each(&blk)
    end
  end
end

application_record.rb

class ApplicationRecord < ActiveRecord::Base
  # ...
  
  def decorate
    "#{self.class.name}Presenter".constantize.decorate(self)
  end

  def self.decorate
    "#{name}Presenter".constantize.decorate(self)
  end

  # ...
end