title: Mongoose - find by referenced field _id
body: <p><strong>Collection Schema</strong></p>
  <pre><code>const notificationsSchema = new mongoose.Schema({
    content: {
      type: String,
      required: true,
      maxlength: 1000
    },
    recipient: {
      type: mongoose.Schema.Types.ObjectId,
      ref: 'User',
      required: true
    }
   }
  });
  </code></pre>
  <p><strong>Corresponding database entry</strong></p>
  <pre><code>{
    &quot;_id&quot; : ObjectId(&quot;607c1ebc3c2e16b610d74464&quot;),
    &quot;content&quot; : &quot;Test Content&quot;,
    &quot;recipient&quot; : &quot;607c1e2c0bb25343e53abf45&quot; &lt;--- Existing _id of a user
  }
  </code></pre>
  <p><strong>Trying to find by recipient field</strong></p>
  <pre><code>deleteUser: combineResolvers(async (_, { id }) =&gt; {
        try {
          const user = await User.findById(id);
          console.log(user.id); &lt;-- returns 607c1e2c0bb25343e53abf45
          
          // Option 1
          const notification = await Notification.findOne({ recipient: user.id }); 
          console.log(notification); &lt;-- returns null
  
          // Option 2
          const userId = mongoose.Types.ObjectId(user.id);
          const notificationObjectId = await Notification.findOne({ recipient: userId }); 
          console.log(notificationObjectId); &lt;-- returns null
  
          // Working alternative
          const notificationAlternative = await Notification.findOne({ content: &quot;Test Content&quot; });
          console.log(notificationAlternative); &lt;-- returns the correct document
  
          return true;
        } catch (error) {
          throw error;
        }
      })  
  </code></pre>
  <p>Just to clarify, I am not trying to find the user by the recipient field, I am trying to find the notification document by the recipient field.
  Why can I not retrieve the document by the recipient id? What am I missing?</p>