Counter cache with foreign key

Neil Cauldwell wrote:

Can I use the Rails counter_cache helper in the following situation;

# Update.rb
has_many :replies, :class_name => "UpdateReply", :dependent => :destroy

# UpdateReply.rb
belongs_to :update, :counter_cache => true

At the moment, I'm getting errors, presumably because of the way the
association is built on a different class name (I have counter caches
without the class name difference working just fine). Do I need to make
my own counter cache for this or can I make the Rails helper work
somehow?

No ideas on this one?