old_contents: import datetime
  
  from django.db import models
  
  
  class OnCall (models.Model):
      rule = models.TextField(
          verbose_name='on-call rule',
          help_text='Python statement for determining the on-call admin')
  
      def get_on_call(self, now=None):
          if now is None:
              now = datetime.datetime.now()
          exec(self.rule)
  
new_contents: import datetime
  
  from django.db import models
  
  
  class OnCall (models.Model):
      slug = models.SlugField(
          verbose_name='on-call slot',
          help_text='Identify this among other possible on-call positions')
      rule = models.TextField(
          verbose_name='on-call rule',
          help_text='Python statement for determining the on-call admin')
  
      def get_on_call(self, now=None):
          if now is None:
              now = datetime.datetime.now()
          exec(self.rule)
  
subject: Add OnCall.slug to support different positions