Can't do multiple groups in MongoDB C# Driver
<p>I am trying to show a statistics count, per month, per manufacturer from a MongoDB collection using Linq.I would like to see my data as:</p>

<pre><code>[{
     Manufacturer: '',
     Statistics: [{
          Month: '',
          Counts: 0
     }]
}]
</code></pre>

<p>So I am trying the following linq query:</p>

<pre><code>var result = _statisticsRepository
            .All()
            .GroupBy(g =&gt; g.ManufacturerId)
            .Select(s =&gt; new
            {
                Manufacturer = s.Key,
                Statistics = s
                    .GroupBy(a =&gt; a.Values["DATA_MONTH"])
                    .Select(a =&gt; new
                    {
                        Month = a.Key,
                        Counts = a.Sum(d =&gt; d.Count)
                    })
            });
</code></pre>

<p>When I try this code I get this error:</p>

<pre><code>NotSupportedException: The method GroupBy is not supported in the expression tree: {document}.GroupBy(a =&gt; a.Values.get_Item("DATA_MONTH")).
</code></pre>

<p>Am I doing this the wrong way or is this a limitation I have in the C# driver? Or is there another way to do this?</p>