title: Rails, joining/including on an instance variable
body: <p>Have a controller with action update, that updates <code>@foo</code>. If there is an error I want to add information to the instance variable via join/include. Such as</p>
  
  <pre><code>if @foo.update_attributes(params[:foo])
    format.js
  else
    @foo = @foo.includes(:other_stuff).joins(:more_stuff)
    format.js
  end
  </code></pre>
  
  <p>That doesn't work (the @foo = @foo....). Is there a way to get this to work? ie get the error codes and params to be entered into the variable.</p>