title: Mongoengine Query using only() on list of reference fields
body: <p>I have two Documents </p>
  
  <pre><code>class Book(Document):
      title = StringField()
      publication = StringField()
      no_of_pages = IntField()
      desc = StringField()
      reviews = ListField(ReferenceField(Review))
      ...
  
  class Author(Document):
      name = StringField()
      books = ListField(ReferenceField(Book))
      desc = StringField()
      ...
  </code></pre>
  
  <p>I want to write a query which given an author's name finds all his book's tiles.</p>
  
  <p>As you can see both the documents have loads of columns, I don't want to get 
  all the columns. hence my required fields are </p>
  
  <pre><code>`req_fields = ['name', 'books']`
  </code></pre>
  
  <p>and I query this as follows</p>
  
  <pre><code>author = Author.objects.filter(name='xyz').only(*req_fields)
  </code></pre>
  
  <p>If I print <code>author</code> object, I get:</p>
  
  <pre><code>{
  _id=ObjectId('50fcf9682c345a0427000019'),
  _cls='Author',
  name='xyz',
  books=[DBRef('Book', ObjectId('50fcd9682c441e0427000019')), 
  DBRef('Book', ObjectId('50fcd9682c361e0427000020'))]
  }
  </code></pre>
  
  <p>I would like the query to return a list of string for title rather than a list of book objects. How can I modify this? so that the result objects is like the following </p>
  
  <pre><code>{
  _id=ObjectId('50fcf9682c345a0427000019'),
  _cls='Author',
  name='xyz',
  books=['Title One', 'Title Two']
  }
  </code></pre>