title: Group result set by combination of multiple columns
body: <p>I have table called negotiations like this</p>
  
  <pre><code>id    seller_id  buyer_id property_id
  1        4           190      33123
  2        4           190      33123
  3        5           191      34000
  4        5           191      34000
  5        6           200      35000
  </code></pre>
  
  <p>I can fetch all by doing:</p>
  
  <pre><code>Negotiation.all
  </code></pre>
  
  <p>I want fetch everything, but grouped by <code>seller_id</code>-<code>buyer_id</code>-<code>property_id</code> combination. This in the above example I want to return three groups.</p>
  
  <p>Is this possible in my rails application?</p>