title: Doctrine mongodb-odm aggregation builder query with ObjectId
body: <p>I have the following user document config in yml mapping with id field as MongoId. I have use case that is required me to use aggregation builder to project data. When the result is available, the document _id will return as MongoId Object. </p>
  
  <p>Does anyone know how to project the _id field in the to return string instead of MongoId?</p>
  
  <p>Document mapping in YML:</p>
  
  <pre><code>UserDocument:
      fields:
          _id:
              id: true
          username:
              type: string
  </code></pre>
  
  <p>Aggregation Query:</p>
  
  <pre><code>$ab = $dm-&gt;createAggregationBuilder('UserDocument');
  $ab-&gt;project()
     -&gt;includeFields([
         'username',
     ]);
  $users = $ab-&gt;execute();
  </code></pre>
  
  <p>Result:</p>
  
  <pre><code>{
    "_id": [],
    "username": "user"
  }
  </code></pre>
  
  <p>Thanks</p>