old_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.  '
              "Store the selected admin as 'on_call'."))
  
      def get_on_call(self, now=None):
          if now is None:
              now = datetime.datetime.now()
          exec(self.rule)
          if 'on_call' in locals():
              return on_call
  
new_contents: import datetime
  
  from django.db import models
  
  
  class OnCall (models.Model):
      slug = models.SlugField(
          unique=True,
          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.  '
              "Store the selected admin as 'on_call'."))
  
      def get_on_call(self, now=None):
          if now is None:
              now = datetime.datetime.now()
          exec(self.rule)
          if 'on_call' in locals():
              return on_call
  
subject: Make OnCall.slug unique