getting unique id

I have an app that needs to get a unique ID. Many threads run at the
same time that need one. I would like the IDs to be sequential. When I
need a unique ID I do this:

with transaction.atomic():
max_batch_id =
JobStatus.objects.select_for_update(nowait=False).aggregate(Max('batch_id'))
json_dict['batch_id'] = max_batch_id['batch_id__max'] + 1
status_row = JobStatus(**json_dict)
status_row.save()

But multiple jobs are getting the same ID. Why does the code not work
as I expect? What is a better way to accomplish what I need?

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