HappyMapper class definition puzzler

manifest.xml

<manifest>
<organization>
<item identifier="ITEM-6137565f-18fe-481f-b6ce-a290a354c862" isvisible="true">
  <title>Math</title>
  <item identifier="ITEM-5781bd41-7acd-4373-8ccd-37bd074c2461">
    <title>Grade 07</title>
    <item identifier="ITEM-d7035880-88a5-4501-9865-f6265ba8022c" identifierref="i-179211">
      <title>qti_200535.xml</title>
    </item>
    <item identifier="ITEM-b13ebebd-f942-4ee8-a342-e8c3913c9063" identifierref="i-194469">
      <title>qti_200577.xml</title>
    </item>
  </item>
  <item identifier="ITEM-39ade5bb-b9a4-48f6-ba48-8c14177c1f65">
    <title>Grades 09-12</title>
    <item identifier="ITEM-13bca2de-134c-4ea5-a69c-50678eba65d5" identifierref="i-180165">
      <title>qti_200600.xml</title>
    </item>
    <item identifier="ITEM-4b86431d-be1f-4e17-b718-e2e5c2235e67" identifierref="i-179198">
      <title>qti_200687.xml</title>
    </item>
    <item identifier="ITEM-4145e484-85bb-4885-8f68-3b8dd3532350" identifierref="i-181841">
      <title>qti_200688.xml</title>
    </item>
    <item identifier="ITEM-30255959-d5f9-45ec-affa-83dc3bf0eb12" identifierref="i-189098">
      <title>qti_201128.xml</title>
    </item>
  </item>
  <item identifier="ITEM-84a2b794-104d-4357-af4d-c330b1ce8f16">
    <title>Grade 08</title>
    <item identifier="ITEM-f95d6101-8d87-431d-8da0-8140a05cda98" identifierref="i-179490">
      <title>qti_201790.xml</title>
    </item>
    <item identifier="ITEM-4864e3f7-1d5b-497f-b026-b6a0c7c51ea6" identifierref="i-193275">
      <title>qti_201854.xml</title>
    </item>
  </item>
</item>
</organization>
</manifest>

qti.rb

# I've noodled around with these four definitions:

class Item
  include HappyMapper
  tag 'item'
  element :title, String, :tag => 'title'
end

class Grade
  include HappyMapper
  tag 'item'
  element :title, String, :tag => 'title'
  has_many :items, Item
end

class Subject
  include HappyMapper
  tag 'item'
  element :title, String, :tag => 'title'
  has_many :grades, Grade
end

class Manifest
  include HappyMapper
  tag 'manifest'
  has_one :subject, Subject, :deep => true
end

# This gets me manifest.subject, but manifest.grades includes the 
# 2nd and 3rd levels, and manifest.grades[n].items is [].

# Any help appreciated.

question.txt

This is an Educational Testing Service XML format based on the QTI specification. 
As you can see below, this XML spec nests <item> tags that essentially mean different 
things. In this case, the first <item> tag level below organization is what I would 
term "subject", the second level is what I would term "grade", and the third level is 
the thing I actually reference as an "item". I need to capture the subject and grade 
information, but when I process "items", I only want those ones at the third level.

Ideally, I'd like to be able to reference:

manifest.subject //Math
manifest.subject.grades
manifest.subject.grades[n].items

But I can't figure out how to write the class definitions for HappyMapper use, 
because everything's an <item>. The different <item> tags have attributes that 
appear to reliably identify the pieces, but not sure how to use that to my 
advantage.