title: Advanced GroupBy
body: <p>I have a List of items with following structure:</p>
  
  <ol>
  <li>Id</li>
  <li>DateTime</li>
  <li>Weight</li>
  </ol>
  
  <p>How can group them by <code>Id</code> and <strong>Date</strong> of <code>Date</code> field (which is <strong>DateTime</strong>) and have the total weight of the group? I want something like this:</p>
  
  <ol>
  <li>Id</li>
  <li>Date</li>
  <li>TotalWeight</li>
  </ol>