Rails - Oct 2005 - uniqueness validation with single table inheritance

I''m experimenting with RoR and am stuck on the following problem.

I have a table called Connections. Connections belong to a Network and have a
name and destination. I am using Ruby''s single table inheritance as
there are
different types of connections (Http connections require a url, LU 6.2
connections require LU names, etc). The Connection names must be unique for the
Network: name and network_id must be unique (this is enforced by the DBMS).

My models for Connection, Http, Lu62 look like:

class Connection < ActiveRecord::Base
   belongs_to   :network

   validates_uniqueness_of :name, :scope => "network_id"
   validates_presence_of   :destination
end

class Http < Connection
   validates_presence_of   :http_url
end

class Lu62 < Connection
   validates_presence_of   :lu_name
end

The problem is that the validates_uniqueness_of only seems to work for
connections of the same type. It catches attempts to insert a duplicate
name-network_id when the two connections are Http, but fails if the duplicate is
of a different type (Lu62).

Am I doing something wrong?

Thanks,
Dan