title: Django queryset annotate field to be a list/queryset
body: <p>I'm trying to use django annotation to create queryset field which is a list of values of some related model attribute.</p>
  
  <pre><code>queryset = ...
  qs = queryset.annotate(
      list_field=SomeAggregateFunction(
          Case(When(related_model__field="abc"), then="related_model__id")
      ),
      list_elements=Count(F('list_field'))
  )
  </code></pre>
  
  <p>I was thinking about about concatenating all these id with some separator, but i don't know the appropriate functions. Another solution is to make list_field a <code>queryset</code>. I know this syntax is wrong. Thank you for any help.</p>