Append class if condition is true in Haml

.post{:class => ("gray" unless post.published?)}

- classes = ["post", ("gray" unless post.published?)]
= content_tag :div, class: classes do
  /Post stuff

---

def post_tag post, &block
  classes = ["post", ("gray" unless post.published?)]
  content_tag :div, class: classes, &block
end

= post_tag post
  /Post stuff

Really the best thing is to put it into a helper.

%div{ :class => published_class(post) }

#some_helper.rb

def published_class(post)
  "post #{post.published? ? '' : 'gray'}"
end