Django QuerySet how to...

1. How to replace EXIST raw SQL with QuerySet.filter() call

class Order(models.Model):
  pass
  
class Item(models.Model):
  order = models.ForeignKey(Order, related_name='items')

How to get empty orders:

Order.objects.extra(where=["NOT EXISTS (SELECT * FROM item WHERE order_id = order.id)"])

Another option, produces query with LEFT OUTER JOIN:

Order.objects.filter(items__isnull=True)

2. How to add relation table into QuerySet SQL query

How to add relation table into resulting query for QuerySet: add related field in the select_related() call:

Item.objects.select_related('order')

The resulting SQL contains the order table JOIN.

3. How to make union of 2 QuerySets for different models

Use values_list() with same count of columns, columns at same positions should be of same type:

Car.objects.values_list('price', 'quantity').union(
  Bicycle.objects.values_list('price', 'quantity')
)