Fix DatabaseScheduler behavior when schedule is changed from interval to crontab

[dmitry-mukhin]

ModelEntry was not checking if there is interval value in DB when updating scheduled task from settings dict.
The result was unexpected (and frustrating) behavior when one decides that he need more fine grained control over execution schedule and change only schedule from timedelta to crontab.

I propose to set schedule fields to None prior setting a proper value.

[georgepsarakis]

@dmitry-mukhin could you please explain the call to function _next_id in https://github.com/celery/django-celery/pull/351/files#diff-fb04340bb23860bdd25c44003fd3430eR104 ? It seems undefined.

[dmitry-mukhin]

@georgepsarakis, as you can see the PR was created few years ago. It's possible that _next_id was defined in that scope back then :)

django-celery/djcelery/tests/test_schedulers.py

Line 29 in 8d6dcde

_next_id = lambda: next(_next_id_get)

_next_id should be changed to next(_ids), as far as I understand

see
https://github.com/celery/django-celery/blob/master/djcelery/tests/test_schedulers.py#L105