Django cached queryset.last() returns None object for not None object

Django Internals ORM
1

My code is:

@ttl_cache(maxsize=50, ttl=60 * 60)
def get_query_set():
  return Model.objects.all().order_by('year')

def get_latest():
  cached_qs = get_query_set()
  try:
    return cached_qs.last().year if len(cached_qs) > 0 else None
  except AttributeError:
    print('~~~~~~~~~~~~~~~~~')
    print('CHECK CIRCLECI PROBLEM')
    print(cached_qs)
    print(cached_qs.last())
    print(len(cached_qs))
    for aa in cached_qs:
      print(aa)
      print(type(aa))
      print(dir(aa))
      print(hasattr(aa, 'year'))
      try:
        print(aa.year)
      except Exception:
        print('no attr for aa')
    print(Model.objects.all().order_by('year'))
    for aaa in Model.objects.all().order_by('year'):
      print(aaa)
      print(aaa.year)
    print('~~~~~~~~~~~~~~~~~')
    raise

Without try-except I getting (on get_latest())

AttributeError: 'NoneType' object has no attribute 'year'

When I catch exception it print:

~~~~~~~~~~~~~~~~~
CHECK CIRCLECI PROBLEM
<QuerySet [<Model: Model object (2)>]>
None
1
Model object (2)
<class 'path.models.model.Model'>
['DoesNotExist', 'Meta', 'MultipleObjectsReturned', '__class__', '__delattr__', ..., 'year']
True
2019
<QuerySet []>
~~~~~~~~~~~~~~~~~

So we can see that

  1. this problem is reproducible only at CircleCI build sometimes (I don’t know how to reproduce it at local machine)
  2. cached queryset is not empty
  3. length of cached queryset is 1
  4. queryset object is not None and has attribute year
  5. value of year is 2019
  6. real object is already deleted at DB and does not exist (physically DB table is empty)
  7. non cached queryset is empty (because of ⑥)

I guess that last() trying to re-get object from DB even if queryset is cached (because of queryset lazy behaviour?). But I don’t understand why it exactly happens? And what I should expect from last() output?

I tried to delete Model object from DB manually after cache python queryset, but last() method working as expected and return value properly.

Any ideas?