membership.rb

class Membership < ActiveRecord::Base
	# attr_accessible :title, :body
	
	set_primary_keys :user_id, :group_id
	  
	belongs_to :user
	belongs_to :group

	# Weird that we have to say "belongs_to", but it works.
	belongs_to :role

	validates_presence_of :user, :group
	validates_uniqueness_of :user_id, :scope => :group_id
  
end

role.rb

class Role < ActiveRecord::Base
	
	has_many :memberships
	
	scope :in_group, lambda { |g| joins(:memberships).where(:group_ids => g.ancestors ) }
	
#	scope :groups joins(:memberships).where(:group_ids => [array_of_group_ids])  
	
end

sql

(SELECT `roles`.* FROM `roles` INNER JOIN `memberships` `memberships_roles` ON `memberships_roles`.`role_id` = `roles`.`id` INNER JOIN `memberships` ON `roles`.`id` = `memberships`.`role_id` WHERE `memberships`.`user_id` = 1160 AND `roles`.`group_ids` IN ('HHC', 'NUCC') AND (expiration > '2012-07-01'))