title: does mongoose have an isDirty check?
body: <p>I have a mongoose setup which involves an embedded-schema, lets say: A Blogpost with embedded comments. Comments can be edited by the original publisher as well as by an editor/admin. After adding / editing a comment the entire blogpost is saved. </p>
  
  <p>I have some custom mongoose's 'pre' middleware set up on the embedded comment-schema which automatically sets the lasteditdate for that particular comment. </p>
  
  <p>The thing is that 'pre' is called on EVERY comment on the blogpost, since I call save() on the blogpost. (For other reasons I need to do it like this) . Therefore, I need a way to check which comments have changed (or are new) since they were last saved (as part of the Blogpost overall save()) </p>
  
  <p>The questio: how to check in 'pre' whether a comment has changed or not? Obviously calling <code>this.isNew</code> isn't sufficient, since comments could be edited (i.e: aren't new) as well. </p>
  
  <p>Is there any <code>isDirty</code> or similar that I'm overlooking?   </p>