title: Group by multiple values Underscore.JS but keep the keys and values
body: <p>I'm trying to group the following array with objects:</p>
  
  <pre><code>[ { user_id: 301, alert_id: 199, deal_id: 32243 },
    { user_id: 301, alert_id: 200, deal_id: 32243 },
    { user_id: 301, alert_id: 200, deal_id: 107293 },
    { user_id: 301, alert_id: 200, deal_id: 277470 } ]
  </code></pre>
  
  <p>As you can see it contains user_id and alert_id combinations, which I like to group. So I would like to have the following array:</p>
  
  <pre><code>[ { user_id: 301, alert_id: 199, deals: [32243] },
    { user_id: 301, alert_id: 200, deals: [32243,107293,277470]}]
  </code></pre>
  
  <p>Anyone knows a solution for this? With underscore's GroupBy I can group the values based on one key. But I need to group them, based on the combination user_id AND alert_id, as you can see.</p>
  
  <p>I took a look at <a href="https://github.com/iros/underscore.nest">underscore.nest</a>, but the problem is it creates its own keys.</p>