title: ObjectId is not being matched with string in graphLookup mongodb nodejs
body: <p>My collection is below</p>
  
  <pre><code>const schema=Schema({
  name: { type: String, required: true,maxlength:50 },
  subcategory: [{ type: String, required: true,maxlength:100 }],
  isActive: { type: Boolean, required: true ,default: 1},
  </code></pre>
  
  <p>});</p>
  
  <p>I inserted data like this</p>
  
  <pre><code>{
          "subcategory": [
              "5da084d13a61194bd4982947"
          ],
          "isActive": true,
          "_id": "5da084cb3a61194bd4982946",
          "name": "drinks",
          "__v": 0
      },
      {
          "subcategory": [],
          "isActive": true,
          "_id": "5da084d13a61194bd4982947",
          "name": "coke",
          "__v": 0
      },
      {
          "subcategory": [],
          "isActive": true,
          "_id": "5da084d63a61194bd4982948",
          "name": "fanta",
          "__v": 0
      }
  </code></pre>
  
  <p>Now I am trying to get graphLookup on "subcategory" with "_id" like below</p>
  
  <pre><code> {
              $graphLookup: {
                  from: 'category', // Use the customers collection
                  startWith: '$subcategory', // Start looking at the document's `friends` property
                  connectFromField: 'subcategory', // A link in the graph is represented by the friends property...
                  connectToField: '_id', // ... pointing to another customer's _id property
                  maxDepth: 1, // Only recurse one level deep
                  as: 'connections' // Store this in the `connections` property
              }
          }
  </code></pre>
  
  <p>but it does not give me result in "connection" field within pipline. It gives reult like this</p>
  
  <pre><code>{
          "_id": "5da084cb3a61194bd4982946",
          "subcategory": [
              "5da084d13a61194bd4982947"
          ],
          "isActive": true,
          "name": "drinks",
          "__v": 0,
          "connections": []
      },
      {
          "_id": "5da084d13a61194bd4982947",
          "subcategory": [],
          "isActive": true,
          "name": "coke",
          "__v": 0,
          "connections": []
      },
  </code></pre>
  
  <p>you can see there is nothing in connections. It is empty.</p>