Django bug in deferred fields

installed using:

• python 3.6.4
• Django==2.0.4
• pytz==2018.4

To see the bug in action, run

$ cd $git_root/deferred_fields_bug
$ python ./manage.py test

explanation:

# with a renamed db column, "_field" is in self.__dict__, but "field" is not,
#  def get_deferred_fields(self):
#      """
#      Return a set containing names of deferred fields for this instance.
#      """
#      return {
#          f.attname for f in self._meta.concrete_fields
#          if f.attname not in self.__dict__
#      }
#
# and then field is not saved in .save(), because django _mistakenly_ thinks
# "field" is deferred, so it is ignored during .save() https://github.com/django/django/blob/93331877c81c1c6641b163b97813268f483ede4b/django/db/models/base.py#L712
# ...
#     elif not force_insert and deferred_fields and using == self._state.db:
#         field_names = set()
#         for field in self._meta.concrete_fields:
#             if not field.primary_key and not hasattr(field, 'through'):
#                 field_names.add(field.attname)
# ->      loaded_fields = field_names.difference(deferred_fields)
#         if loaded_fields:
#             update_fields = frozenset(loaded_fields)
#
#     self.save_base(using=using, force_insert=force_insert,
#                    force_update=force_update, update_fields=update_fields)
# ...

# asserting that django thinks "field" is deffered (not that .models does NOT tell it to defer this field!)