title: Simple $match greater than $gt not working
body: <p>This is the result of an aggregate query,</p>
  
  <pre><code>{
      "_id" : ObjectId("5dab3240dfbe9a15cd69771d"),
      "isManual" : false,
      "frequency" : 60,
      "lastExecuted" : ISODate("2019-10-21T03:38:15.114Z"),
      "lastExecutedTimeFromNow" : 129.58105
  }
  {
      "_id" : ObjectId("5dad47c65310a16581cc6294"),
      "isManual" : false,
      "frequency" : 50,
      "lastExecuted" : ISODate("2019-10-25T00:00:00.000Z"),
      "lastExecutedTimeFromNow" : 100
  }
  {
      "_id" : ObjectId("5dad48a55310a16581cc6332"),
      "isManual" : true,
      "frequency" : 100,
      "lastExecuted" : ISODate("2019-10-23T00:00:00.000Z"),
      "lastExecutedTimeFromNow" : 50
  }
  </code></pre>
  
  <p>I wanted to filter the documents where the field <code>lastExecutedTimeFromNow</code> greater than <code>frequency</code>. But it returns 0 results.</p>
  
  <p>Here's the <code>aggregate</code> query I'm using,</p>
  
  <pre><code>db.getCollection('test').aggregate([
      {
          $match: {
              "lastExecutedTimeFromNow": { $gte: "$frequency" }
          }
      }
  ])
  </code></pre>
  
  <p>Any clue on where I'm going wrong or any help on this would really be great.</p>