title: Django Queryset filter checks if related objects exist
body: <p>I'm trying to create a custom manager which return all Foo instances that dont have Bar attached to them.</p>
  
  <pre><code># models.py
  class Foo(models.Model):
      ...
  
  class Bar(models.Model):
      foo = models.OneToOneField(Foo)
      ...
  
  # managers.py
  class FooQueryset(BaseQueryset):
      def no_bar(self):
          return ???
  
  class FooManager(BaseManager):
      def get_queryset(self):
          return EcheanceQueryset(self.model, using=self._db)
  
      def no_bar(self):
          return self.get_queryset().no_bar()
  </code></pre>
  
  <p>I'm asking for help with the queryset to get the desired result</p>