Non-working has 1, :through => :model relationship

test-case.rb

require 'data_mapper'
DataMapper::Logger.new($stdout)
DataMapper.setup(:default, 'sqlite::memory:')

class Pack
  include DataMapper::Resource
  belongs_to :school

  has n, :pack_orders
  
  property :id, Serial
end

class PackOrder
  include DataMapper::Resource
  
  belongs_to :pack
  has 1, :school, :through => :pack 
  # I tried :belongs_to but DM throws an 
  # error about using :through with belongs_to
  
  property :id, Serial
end

class School
  include DataMapper::Resource

  has n, :packs
  has n, :pack_orders, :through => :packs
  
  property :id, Serial
end

DataMapper.finalize
DataMapper.auto_migrate!

@school = School.create
@pack = Pack.create :school => @school
@pack_order = PackOrder.create :pack => @pack

# Works
@orders = PackOrder.all :pack => { :school => @school }
@orders.each do |order|
  puts order.id
end

# Doesn't work
@orders = PackOrder.all :school => @school
@orders.each do |order|
  puts order.id
end