:table_name_with_underscore and join models

Per my PHBs, I'm using

ActiveRecord::Base.primary_key_prefix_type
= :table_name_with_underscore

in my current project.

My problem is that this setting seems to be causing me some issues
with my join model's primary keys.

For example I have these three models:

class User < ActiveRecord::Base
  has_many :user_ugroups
  has_many :ugroups, :through => :user_ugroups
end

class Ugroup < ActiveRecord::Base
  has_many :user_ugroups
  has_many :users, :through => :user_ugroups
end

class UserUgroup < ActiveRecord::Base
  belongs_to :user
  belongs_to :ugroup
end

My migration for the join table looks like this:

def self.up
  create_table :user_ugroups, :primary_key => :user_ugroups_id do |t|
    t.integer :user_id, :null => false
    t.integer :ugroup_id, :null => false
  end
end

So when I create a UserUgroup like so:

UserUgroup.create!( :user => @gdonald, :ugroup => @admin )

ActiveRecord tries to insert a NULL for the user_ugroups_id field and
then I get this:

ActiveRecord::StatementInvalid: OCIError: ORA-01400: cannot insert
NULL into ("MYPROJDB"."USER_UGROUPS"."USER_UGROUPS_ID"): INSERT INTO
user_ugroups (user_ugroups_id, user_id, ugroup_id) VALUES(NULL, 1, 1)

The migration creates my USER_UGROUPS_SEQ without issue, but then it
doesn't get used.

Any ideas?

I forgot to mention, I also tried

set_sequence_name "USER_UGROUPS_SEQ"

in my join model, but I got the same error.

For the next poor Oracle user who comes along, here's a workaround:

class UserUgroup < ActiveRecord::Base

  before_save :get_id

  belongs_to :ugroup
  belongs_to :user

  def get_id
    self.user_ugroup_id = connection.next_sequence_value(
self.class.sequence_name )
  end

end

Oops.. that should be before_create