title: how to group an array of objects based on two properties
body: <p>I would like to get an array of arrays like ...[[1,2],[5,8],[9]]... depending on the group by.</p>
  
  <p>This is the example I am following from the rxjs library example. This ex only uses one property to group by.</p>
  
  <pre><code>Observable.of&lt;Obj&gt;({id: 1, age: 5, weight: 145},
                     {id: 9, age: 7, weight: 10},
                     {id: 2, age: 5, weight: 145},
                     {id: 60, age: 11, weight: 145},
      )
      .groupBy(p =&gt; p.id)
      .flatMap( (group$) =&gt; group$.reduce((acc, cur) =&gt; [...acc, cur], []))
      .subscribe(p =&gt; console.log(p));
  </code></pre>
  
  <p>Expected result:    [[1,2],[9],[60]]</p>
  
  <p>I haven't managed to make it work with more than one property (age and weight).</p>
  
  <p>Could anybody tell me how I can achieve this?</p>
  
  <p>Thanks,</p>