I ran into an issue today.  I have a Blog model that has_many feed_entries.

In plain english, I needed a list of blogs ordered by most recently updated feed.

My models look like this:

blog.rb
has_many :feed_entries

feed_entry.rb
belongs_to :blog

The query that makes it possible:

Blog.find(:all, :include => :feed_entries, :order => 'feed_entries.published_at DESC')

:include makes this possible!