def something
  @foo = Foo.find_by_guid(params[:id])
  unless @foo
    @msg ||= { :title => 'No such page!',
      :desc => "There is no such page!" }
    render :action => "error" and return
  end
  unless @foo.owner_id == current_user.id
   @msg ||= { :title => 'Really?',
      :desc => "There is no such page." }
    render :action => "error" and return
  end

class MegaController < ActionController::Base
    before_filter :grab_and_check_foo
    #...
private
    def grab_and_check_foo
        @foo = Foo.find_by_guid(params[:id])
        if !@foo
            #... render some error stuff
        end
        if @foo.owner_id != current_user.id
            #... render some other error stuff
        end
    end
end