Haml: Append class if condition is true in Haml

If post.published?

.post
  / Post stuff

Otherwise

.post.gray
  / Post stuff

I've implemented this with rails helper and it seems ugly.

= content_tag :div, :class => "post" + (" gray" unless post.published?).to_s do
  / Post stuff

Second variant:

= content_tag :div, :class => "post" + (post.published? ? "" : " gray") do
  / Post stuff

Is there a more simple and haml-specific way?

UPD. Haml-specific, but still not simple:

%div{:class => "post" + (" gray" unless post.published?).to_s}
  / Post stuff