title: Odata The method 'GroupBy' is not supported
body: <p>Using the devextreme below code to get the sum of amount however as I am using Odata, it throws exception of "The method 'GroupBy' is not supported". Yet there is no answer for this mystery. Is there any suggestion how to get the total amount in any other way?</p>
  
  <pre><code>   .Summary(s =&gt; s                                                                   
     .TotalItems(totalItems =&gt;
  
      {
  
     totalItems.AddFor(m =&gt; 
     m.MassAmount).Column("MassAmount").SummaryType(SummaryType.Sum);
  
      })
  
    [HttpGet]
      public HttpResponseMessage Get(DataSourceLoadOptions loadOptions)
     {
        return 
       Request.CreateResponse(DataSourceLoader.Load(_db.Txn, 
        loadOptions));
      }
  </code></pre>