Django - Как вычесть две аннотации с разными моделями

a = Category.objects.annotate(total_products=Count('product'))
b=Order.objects.values('category__name').annotate(total_approve=Sum('approve_quantity')).order_by('-total_approve')

поэтому я хочу вычесть a - b.

Обе аннотации работают нормально, но как уменьшить total_products в категории, когда определенное количество утверждено, является проблемой. Есть ли другой способ?

class Category(models.Model):
    name = models.CharField(max_length=50, blank=True, null=True)
    reorder_level = models.IntegerField(default='0', blank=True, 
        null=True)
    timestamp = models.DateTimeField(auto_now_add=False, auto_now=True, 
        null=True)

class Product(models.Model):
    pro_name = models.CharField(max_length=100, blank=True, null=True)
    category = models.ForeignKey(Category, on_delete=models.CASCADE, blank=True, null=True)
    order = models.ForeignKey('Order',default='', on_delete=models.CASCADE,blank=True, null=True)
    serial_num = models.CharField(max_length=100, blank=True, null=True)
    model_num = models.CharField(max_length=100, blank=True, null=True)
    storage_size = models.CharField(max_length=50, blank=True, null=True)
    memory_size = models.CharField(max_length=50, blank=True, null=True)
    supplier = models.ForeignKey(Supplier, on_delete=models.CASCADE, blank=True, null=True)
    receive_quantity = models.IntegerField(default='0', blank=True, null=True)
    issue_by = models.CharField(max_length=50, blank=True, null=True)
    issue_to = models.CharField(max_length=50, blank=True, null=True)
    last_updated = models.DateTimeField(auto_now_add=False, auto_now=False, null=True) 
    timestamp = models.DateTimeField(auto_now_add=False, auto_now=True, null=True)

class Order(models.Model):
    category = models.ForeignKey(Category, on_delete=models.CASCADE, 
        null=True)
    pro_name = models.ForeignKey(Product, on_delete=models.CASCADE, 
        null=True,related_name='product') 
    staff = models.ForeignKey(User, on_delete=models.CASCADE, null=True)
    order_quantity = models.PositiveIntegerField(null=True)
    department = models.CharField(max_length=50, choices=DEPARTMENT, null=True)
    order_status = models.IntegerField(default=0)
    approve_quantity = models.IntegerField(default='1', blank=True, null=True)
    transaction_id = models.CharField(default=uuid.uuid4().hex[:8].upper(), max_length=50, editable=False)
    timestamp = models.DateTimeField(auto_now_add=False, auto_now=True, null=True)