# encoding: utf-8
module ActiveMerge

  # Service Object responds for merging two ActiveRecord instances as a whole
  # transaction.
  #
  # All objects linked to the second instance via "has_many" association
  # are re-associated to the first one.
  # 
  # Then the second instance removed.
  # All errors collected in the #errors method.
  #
  class SimpleService < ActivePatterns::BaseService
    include ActiveModel::Validations

    def initialize(first, second)
      @klass  = first.class
      return unless klass.ancestors.include?(ActiveRecord::Base) &&
        second.is_a?(klass) && first.persisted? && second.persisted?
      @first, @second = first, second
      @links  = find_links
    end
    attr_reader :klass, :first, :second, :links

    validates :first, :second, presence: true

    def provide(validate: true)
      transaction do
        remove_second! validate
        save_links! validate
      end
    end

    private

      # Finds links to the second record and rebinds it to the first one.
      # Doesn't validate and save links yet!
      def find_links
        return [] unless first && second
        klass.reflect_on_all_associations(:has_many).inject([]) do |arr, item|
          arr + second.send(item.name).map do |link|
            link.send "#{ item.foreign_key }=", first.id
            link
          end
        end
      end

      # Removes the second record.
      # Any errors are moved to the service.
      def remove_second!(validate)
        begin
          validate ? second.destroy! : second.delete
        rescue => error
          second.errors.each{ |_, message| errors.add :second, message }
          raise error
        end
      end    

      # Saves changed links.
      # Any errors are moved to the service.
      def save_links!(validate)
        links.each do |link|
          begin
            validate ? link.save! : link.save(validate: false)
          rescue => error
            link.errors.each{ |_, message| errors.add :base, message }
            raise error
          end
        end
      end
  end
end