One of my controllers has close to 100 methods (eg routes) and nearly every one starts out the same code to redirect to an error page if the id param is invalid, followed by a similar check if the user that id doesn't belong in the user's account:

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

What is the best way to DRY up that sort of page id and owner id validation, given the code is doing a render ... and return?

What I don't want to do at this point is offload it to a blackbox roles and permissions library like CanCan... my goal is simply to have the in-app code to handle this be as clean as possible.