title: Mongodb 3.6.0-rc3 array filters not working?
body: <p>I'm trying to use array filters in mongodb 3.6.0-rc3, exactly like in <a href="https://docs.mongodb.com/master/reference/operator/update/positional-filtered/#update-all-array-elements-that-match-arrayfilters" rel="nofollow noreferrer">doc example</a> but not getting any rows affected and no error.</p>
  
  <p>Example is simplified. I know this can be done with $ as positional operator but I'm planing to use this feature for two level nested arrays.</p>
  
  <pre><code>db.getCollection('books').update({},
      {
          $set: { "authors.$[element].firstName": "Joe" }
      },
      {
          arrayFilters: [ { element: { "_id": ObjectId("some_id") } } ],
          multi: true
      })
  </code></pre>
  
  <p>Anyone tried this yet?</p>