title: rails touch chain with has_many :through associations
body: <p>Using a join model, with a <code>has_many :through</code> on this example setup</p>
  
  <pre><code>class Collage
   has_many :arrangements
   has_many :photos, through: :arrangements
  
  class Photo
   has_many :arragements
   has_many :collages, through: :arragements
  end
  
  class Arragement
  belongs_to :photo
  belongs_to _collage
  end
  </code></pre>
  
  <p>The photo may change it size, which will cause the collage to change</p>
  
  <p>Using the <code>touch: true</code>, doesn't work this way, because the chain isn't "one way up", as <code>arragement</code> points to <code>Photo</code> and <code>Collage</code></p>
  
  <p>How can I work this so a Photo change (ie touch) would also touch its Collages?</p>