title: Mongodb: Unsupported conversion from array to objectId in $convert with no onError value
body: <p>I use mongodb 4.0.5. I'm using lookup to join two collections, the foreign key is a string value located in request_by array and the other one is 
  ObjectId</p>
  
  <pre><code>{
   $addFields: {
              convertedId: {
                  $toObjectId: "$request_by.userId"
              }
          }
  }
  </code></pre>
  
  <p>i want to convert the foreign key to ObjectId so i can join them. But it says "Unsupported conversion from array to objectId in $convert with no onError value"</p>
  
  <p>i have a data something like this:</p>
  
  <pre><code>Simulation collection
      {
          "_id": "8f361e8969948e1c435c06d7",
          "request_by": [{
              "userId": "ae83ccfa592f4963a395263c",
              "iat": 1544801930,
              "exp": 1544819930
          }],
          "status": "finish",
          "start": "2018-12-14T15:39:29.588Z",
          "end": "2018-12-14T16:59:29.538Z",
          "duration": 80,
          "passing_grade": 100,
          "created_at": "2018-12-14T15:39:29.588Z",
          "updated_at": "2018-12-14T15:43:12.897Z",
          "__v": 0
      }
  </code></pre>
  
  <p>How i can join them if i have data like that?</p>