title: Mongoengine raw find_and_modify query gives "Must either update or remove" exception
body: <p>I am using Django + mongoengine
  I want to update the books (embedded document) count in my Bookscollection document. I want the update query to return the full object. Therefore I am using the 'find_and_modify'. But even if I use update or remove fields. I still get an error that 'Must either update or remove'.
  Here is my code snippet - </p>
  
  <pre><code>for book_id in book_list:
          book_col = BookCollection._get_collection().find_and_modify({
                          'query': {'coll_id':book_coll_id,'book.book_id':book_id},
                          'update':{'$inc':
                               {'book.$.quantity' : 1}
                           },
                          'new':True
          })
  </code></pre>
  
  <p>What is wrong here?</p>