Mongoengine query using only () on a list of referenced fields

I have two documents

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()
    ...

      

        
        
        
      

    

I want to write a query that gives the name of the author, finds all of his book tiles.

As you can see there are many columns in both documents, I don't want to get all columns. so my required fields are

`req_fields = ['name', 'books']`

      

        
        
        
      

    

and I am requesting it like this

author = Author.objects.filter(name='xyz').only(*req_fields)

      

        
        
        
      

    

If I print an object author

, I get:

{
_id=ObjectId('50fcf9682c345a0427000019'),
_cls='Author',
name='xyz',
books=[DBRef('Book', ObjectId('50fcd9682c441e0427000019')), 
DBRef('Book', ObjectId('50fcd9682c361e0427000020'))]
}

      

        
        
        
      

    

I would like the query to return a list of strings for the title, not a list of book objects. How can I change this? so the result objects look like this:

{
_id=ObjectId('50fcf9682c345a0427000019'),
_cls='Author',
name='xyz',
books=['Title One', 'Title Two']
}