title: Scope/query based on a specific instance of an has_many associations
body: <p>Let's say I have a <code>Person</code> class with many <code>transaction_records</code>.</p>
  
  <p>How do I retrieve all the Person which have their last <code>transaction_record</code>(based on <code>created_at</code>) having a <code>created_at</code> attribute older than 14 days.</p>
  
  <p>I'd preferably use ActiveRecord methods (vs long SQL query)</p>