Internal Server Error: /c/2a_rozne/admin/contests/contest/2a_rozne/delete/
Traceback (most recent call last):
  File "/home/users/sio2/sio2/venv/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 132, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/home/users/sio2/sio2/venv/local/lib/python2.7/site-packages/django/utils/decorators.py", line 145, in inner
    return func(*args, **kwargs)
  File "/home/users/sio2/sio2/venv/local/lib/python2.7/site-packages/django/db/transaction.py", line 225, in __exit__
    connection.commit()
  File "/home/users/sio2/sio2/venv/local/lib/python2.7/site-packages/django/db/backends/base/base.py", line 173, in commit
    self._commit()
  File "/home/users/sio2/sio2/venv/local/lib/python2.7/site-packages/django/db/backends/base/base.py", line 142, in _commit
    return self.connection.commit()
  File "/home/users/sio2/sio2/venv/local/lib/python2.7/site-packages/django/db/utils.py", line 98, in __exit__
    six.reraise(dj_exc_type, dj_exc_value, traceback)
  File "/home/users/sio2/sio2/venv/local/lib/python2.7/site-packages/django/db/backends/base/base.py", line 142, in _commit
    return self.connection.commit()
IntegrityError: update or delete on table "problems_problem" violates foreign key constraint "gamification_problem_id_6c405b0a31bad73a_fk_problems_problem_id" on table "gamification_problemdifficulty"
DETAIL: Key (id)=(11080) is still referenced from table "gamification_problemdifficulty".

The solution is to add ON CASCADE DELETE in the Django models for gamification.