title: Ohm & Redis: when to use set, list or collection?
body: <p>What is the difference between a collection and a set or list when using Ohm &amp; Redis?</p>
  
  <p>Several of the Ohm examples use a list rather than a collection (see the <a href="http://ohm.keyvalue.org/Ohm/Model.html#list-class_method">list doc itself</a>):</p>
  
  <pre><code>class Post &lt; Ohm::Model
    list :comments, Comment
  end
  
  class Comment &lt; Ohm::Model
  end
  </code></pre>
  
  <p>What is the rationale for this design choice? </p>