title: MongoError: Unrecognized pipeline stage name: '$expr' on using $divide and $expr in aggregation
body: <p>New to Mongodb.</p>
  
  <p>What I'm attemping to do is create a formula that returns documents if the rent value is $gte to 1% of the price value.</p>
  
  <p>Right now I have both price and rent values stored in the database but not the 1% value.</p>
  
  <p>I tried the code below but I get this error 
  <code>MongoError: Unrecognized pipeline stage name: '$expr'</code></p>
  
  <p>Here's my code</p>
  
  <pre><code>agg = [
          {
              "$set": {
                  "onePercent": { '$divide': ["$price", 100] }
              }
          },
          {
              "$expr": {
                  $gte: ["$rent", "$onePercent",]
              }
          },
      ]
      let res = client.db('myDb').collection('myCollection').aggregate(agg)
  </code></pre>
  
  <p>How can I make a query that would return the documents I'm attempting to select?</p>