title: Deleting document doesn't work in a mongoose pre('remove') hook
body: <p>I am trying to delete a Book which has references to the chapters. When I delete the book, I want all the related chapters removed along with it. But I have been unsuccessful in removing the chapter documents from the DB. Here is how I am removing it.  </p>
  
  <p>First is the controller method:</p>
  
  <pre><code>exports.delete = async(req, res) =&gt; {
    const book = await Book.findOne({ id: req.params.id});
    await book.remove();
    res.redirect(`/`);
  }
  </code></pre>
  
  <p>After that, the <code>pre('remove')</code> hook gets triggered:</p>
  
  <pre><code>bookSchema.pre('remove', function(next){
    Chapter.findOneAndRemove({book_id: this.id});
    next();
  });
  </code></pre>
  
  <p>I have also tried <code>document.remove()</code> and <code>Model.deleteMany()</code> but the documents are still there. There are no errors and the hook middleware is triggered. I realize that in the code example I am only removing one document instead of documents but even that does not work. The book gets deleted from the DB but the chapters remain.</p>