title: How to always update updated_at of model when associations are mutated
body: <p>I have a model like:</p>
  
  <pre><code>class Company &lt; AR
    has_many :locations
  end
  
  class Locations &lt; AR
    has_many :stores
  end
  
  class Store &lt; AR
    has_many :products
  end
  </code></pre>
  
  <p>Is it possible to have the company.updated_at to be updated each time any of the associations are mutated?</p>
  
  <p>e.g. if a product is updated, I want it to bubble up and updated the company.updated_at column.</p>
  
  <p>Is this possible?</p>
  
  <p>Along the same lines, can I hook into the event when the company.updated_at is updated so I can make a redis call?</p>