django - Using Q object in list comprehension -

i have following code:

def get_elements(self, obj):   book_elements = element.objects.filter(book__pk=obj.id)   elements = element.objects.filter( (q(book__pk=obj.id) | q(theme__pk=obj.theme_id)), ~q(pk__in = [o.element_id o in book_elements if o.element_id]))   serializer = getelementserializer(elements, context=self.context, many=true) 

the elements variable query using q object implementation. however, q(book__pk=obj.id) , book_elements reference exact same set of values. how can reference q(book__pk=obj.id) inside of list comprehension avoid having run 2 queries. following:

def get_elements(self, obj):   elements = element.objects.filter( (q(book__pk=obj.id) | q(theme__pk=obj.theme_id)), ~q(pk__in = [o.element_id o in q(book__pk=obj.id) if o.element_id]))   serializer = getelementserializer(elements, context=self.context, many=true) 

my element model requested:

class element(models.model):   name = models.charfield(max_length=100)   pub_date = models.datetimefield('date published', auto_now_add=true)   mod_date = models.datetimefield('modified date', auto_now=true)   book = models.foreignkey(book, blank=true, null=true, related_name='elements')   book_part = models.foreignkey(bookpart)   theme = models.foreignkey(theme, blank=true, null=true, related_name='elements')   element = models.foreignkey('self', blank=true, null=true, related_name='parent')   def __str__(self):     return self.name 

any appreciated!

here 1 query sub-query solution:

elements = element.objects.filter(      (q(book__pk=obj.id) | q(theme__pk=obj.theme_id)),     ~q(pk__in = element.objects.filter(             book__pk=obj.id,             element__isnull=false         ).values_list('element', flat=true)     ) ) 

this hit database once.

btw, after reading trying achieve:

get me elements, book_id or theme_id, equals book id & theme id book, excluding elements have fk relationship element in book

in orm fashion think can achieved this:

elements = element.objects \     .filter(q(book_id=book.id)|q(theme_id=book.theme_id)) \     .exclude(element__in=book.elements.all()) #not tested, if throws error transform into: book.elements.values_list('id', flat=true) 

again hit database once, because querysets lazy , django smart enough transform them sub-query when take apart queryset.