old_contents: from django.db import models
  
  class Role(models.Model):
      name=models.CharField(max_length=30)
      def __unicode__(self):
          return self.name
  
new_contents: from django.db import models
  
  class Role(models.Model):
      name=models.CharField(max_length=30, unique=True)
  
      def __unicode__(self):
          return self.name
  
subject: Set role names to be unique