title: Matching ObjectId to String for $graphLookup
body: <p>I'm trying to run a <code>$graphLookup</code> like demonstrated in print bellow:</p>
  
  <p><a href="https://i.stack.imgur.com/ibSz4.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ibSz4.png" alt="$graphLookup sample"></a></p>
  
  <p>The objective is to, given a specific record (commented <code>$match</code> there), retrieve it's full "path" throught <code>immediateAncestors</code> property. As you can see, it's not happening.</p>
  
  <p>I introduced <code>$convert</code> here to deal with <code>_id</code> from collection as <code>string</code>, believing it could be possible to "match" with <code>_id</code> from <code>immediateAncestors</code> records list (which is a <code>string</code>).</p>
  
  <p>So, I did run another test with different data (no <code>ObjectId</code>s involved):</p>
  
  <pre><code>db.nodos.insert({"id":5,"name":"cinco","children":[{"id":4}]})
  db.nodos.insert({"id":4,"name":"quatro","ancestors":[{"id":5}],"children":[{"id":3}]})
  db.nodos.insert({"id":6,"name":"seis","children":[{"id":3}]})
  db.nodos.insert({"id":1,"name":"um","children":[{"id":2}]})
  db.nodos.insert({"id":2,"name":"dois","ancestors":[{"id":1}],"children":[{"id":3}]})
  db.nodos.insert({"id":3,"name":"três","ancestors":[{"id":2},{"id":4},{"id":6}]})
  db.nodos.insert({"id":7,"name":"sete","children":[{"id":5}]})
  </code></pre>
  
  <p>And the query:</p>
  
  <pre><code>db.nodos.aggregate( [
    { $match: { "id": 3 } },
    { $graphLookup: {
        from: "nodos",
        startWith: "$ancestors.id",
        connectFromField: "ancestors.id",
        connectToField: "id",
        as: "ANCESTORS_FROM_BEGINNING"
      }
    },
    { $project: {
        "name": 1,
        "id": 1,
        "ANCESTORS_FROM_BEGINNING": "$ANCESTORS_FROM_BEGINNING.id"
      }
    }
  ] )
  </code></pre>
  
  <p>...which outputs what I was expecting (the five records directly and indirectly connected to the one with <code>id</code> 3):</p>
  
  <pre><code>{
      "_id" : ObjectId("5afe270fb4719112b613f1b4"),
      "id" : 3.0,
      "name" : "três",
      "ANCESTORS_FROM_BEGINNING" : [ 
          1.0, 
          4.0, 
          6.0, 
          5.0, 
          2.0
      ]
  }
  </code></pre>
  
  <p>The question is: there is a way to achieve the objetive I mentioned in the beginning?</p>
  
  <p>I'm running Mongo 3.7.9 (from official Docker)</p>
  
  <p>Thanks in advance!</p>