title: Updating a value that is dependent on a newly updated document key
body: <p>My goal is to add a comment to my CommentFeed and while doing that I want to push that comment into my <code>topComments</code> field and also update the 'numOfComments' . I want to limit the <code>topComments</code> to only 3 comments (How would I even set that up?). And how do I take the previous value of <code>numOfComments</code> and add one to it? </p>
  
  <pre><code> CommentFeed.findOneAndUpdate(
              { _id: commentId },
              {
                  $push: {
                      comments: {
                          text: req.body.text
                      },
                   $push: topComments:{text: req.body.text}, &lt;--- Limit this somehow to only allow an array length of 3?
                   $set: numOfComments: ? ,  &lt;---What kind of logic is used here?
                  }
              },
              { new: true }
          )
  </code></pre>
  
  <p><strong>CommentFeed Schema</strong></p>
  
  <pre><code>    const CommentFeedSchema = new Schema({
                topComments:[{text:{type:String}}],
                numOfComments:{type:Number},
                comments: [
                     text: { type: String, required: true }
                      ]});
  </code></pre>