title: Why mongo show data object ID as string ID
body: <p>I store Event collection as</p>
  <pre><code>const Event = new Schema({
    creator: { type: ObjectId, ref: &quot;_User&quot;, required: true, index: true }
  })
  </code></pre>
  <p>When I extract event data, some creators show as User Object. But some creators show as string &quot;612267352379704de4c5552e&quot;. I want only object creators. Why do creators change from object to string.</p>