title: Check if field is modified in mongoose's post hook for function findOneAndUpdate
body: <p>In the controller I'm incrementing a field with findOneAndUpdate. The increment is not necessarily always executed, which depends on whether the condition for a update query has been met.
  If the field has been incremented tho, an another model should updated as well. So, I was thinking to use:</p>
  
  <pre><code>schema.post('findOneAndUpdate', function(err) {
      pseudo-code:
      if (field is incremented)
        anotherModel.update()
  });
  </code></pre>
  
  <p>The problem is that I don't seem to be able to determine if the field has been modified.
  Self reference (this) points to a funny object of considerable size, (unlike in e.g. middleware functions for 'save'), so I don't see much other options to determine if field is modified.</p>
  
  <p>Any ideas?</p>