visitor.rb

module Visitable
  def accept(visitor)
    raise 'does not implement error'
  end
end

# class that accept any visitor
class User < ApplicationRecord
  include Visitable

  # some code

  def accept(visitor)
    visitor.visit_user(self)
  end
end

# class that accept any visitor
class Order < ApplicationRecord
  include Visitable

  # some code

  def accept(visitor)
    visitor.visit_order(self)
  end
end


# main class for any visitors
class AbstractVisitor
  def visit_user(user)
    raise 'does not implement error'
  end

  def visit_order(order)
    raise 'does not implement error'
  end

  # here we can add some method for any classes
end


class XMLExportVisitor < AbstractVisitor
  def visit_user(user)
    # export logic user to XML is here
  end

  def visit_order(order)
    # export logic order to XML is here
  end
end

class JSONExportVisitor < AbstractVisitor
  def visit_user(user)
    # export logic user to JSON is here
  end

  def visit_order(order)
    # export logic order to JSON is here
  end
end



# and controller in rails-app
module Api
  module Users
    class ExportsController < ApplicationController
      before_action :find_user

      def xml_export
        @report = @user.accept(XMLExportVisitor.new)
      end

      def json_export
        @report = @user.accept(JSONExportVisitor.new)
      end

      private

      def find_user
        @user = User.find(params[:id])
      end
    end
  end
end