class User
  include DataMapper::Resource

  has n, :roles, :through => Resource
  has n, :permissions, :through => :roles
end

class Role
  include DataMapper::Resource

  has n, :users, :through => Resource
  has n, :permissions, :through => Resource
end

class Permission
  include DataMapper::Resource

  has n, :roles, :through => Resource
  has n, :users, :through => :roles
end

User.permissions