title: FieldPath field names may not contain '.' in $group
body: <p>I have the following mongo data which looks like this</p>
  
  <pre><code>{
      eventType : "mousedown",
      eventArgs : {
          type : "touchstart",
          elementId : "id1"
      },
      creationDateTime : ISODate("2017-02-24T07:05:49.986Z")
  }
  </code></pre>
  
  <p>I wrote the following query to perform group count.</p>
  
  <pre><code>db.analytics.aggregate
  (
      {
          $match :
          {
              $and : 
              [
                  {"eventArgs.type" : 'touchstart'}, 
                  {eventType : 'mousedown'}, 
                  {creationDateTime : {$gte : ISODate("2017-02-24T000:00:00.000Z")}}
              ]
          }
      },
      {
          $group : 
          {
              _id : 
              {
                  "eventsArgs.elementId" : "$elementId"
              },
              count : 
              {
                  $sum : 1
              }
          }
      }
  );
  </code></pre>
  
  <p>I'm getting error for <code>$group</code>, which states that</p>
  
  <pre><code>FieldPath field names may not contain '.'
  </code></pre>
  
  <p>If I were not able to specific '.' in</p>
  
  <pre><code>        $group : 
          {
              _id : 
              {
                  "eventsArgs.elementId" : "$elementId"
              },
  </code></pre>
  
  <p>What is the correct way to do so?</p>