gistfile1.rb

class Group < ActiveRecord::Base
  has_many :memberships, :dependent => :destroy
  has_many :users, :through => :memberships

  named_filter :moderated do
    having(:memberships).with(:state, 'moderator')
  end

  named_filter :with_pending_memberships do
    having(:memberships).with(:state, 'pending')
  end
end


class User < ActiveRecord::Base
  has_many :memberships, :dependent => :destroy
  has_many :groups, :through => :membership
end


class Membership < ActiveRecord::Base
  belongs_to :user
  belongs_to :group
end

> Group.moderated.with_pending_memberships
SELECT "groups".* FROM "groups" INNER JOIN "memberships" AS groups__memberships ON "groups".id = groups__memberships.group_id WHERE ((groups__memberships.state = 'pending') AND (groups__memberships.state = 'moderator'))
=> []