Argh!
I think I found the problem.
When joining via:
@comments = Comment.find(:all,
:joins => "forum_topics",
:conditions => ["forum_topics.featured = ? ", true]
)
You need the whole join!
:joins => "INNER JOIN forum_topics ON forum_topics.id = comments.commentable_id",
See the ever-awesome:
http://guides.rubyonrails.org/active_record_querying.html#joining-tables