old_contents: from django.db import models
  from django.utils.translation import ugettext_lazy as _
  
  class Issue(models.Model):
      title = models.CharField(_('Title'), max_length=150)
      description = models.TextField(_('Description'))
      contract = models.ForeignKey('structure.Contract')
      assigned_team = models.ForeignKey('structure.Team')
      assigned_user = models.ForeignKey('structure.User', null=True)
  
new_contents: from django.db import models
  from django.utils.translation import ugettext_lazy as _
  
  class Issue(models.Model):
      title = models.CharField(_('Title'), max_length=150)
      description = models.TextField(_('Description'))
      contract = models.ForeignKey('structure.Contract')
      assigned_team = models.ForeignKey('structure.Team')
      assigned_user = models.ForeignKey('structure.User', null=True)
      created_at = models.DateTimeField(auto_now_add=True)
  
subject: Add created_at to Issue model