old_contents: from django.db import models
  from django.utils.translation import ugettext_lazy as _
  
  class TaxGroup(models.Model):
  
      name = models.CharField(_("group name"), max_length=100)
      rate = models.DecimalField(_("rate"), max_digits=4, decimal_places=2,
                                 help_text=_("Percentile rate of the tax."))
      rate_name = models.CharField(_("name of the rate"), max_length=30,
                                   help_text=_("Name of the rate which will be"
                                               " displayed to the user."))
      def __unicode__(self):
          return self.name
  
  
  class TaxedProductMixin(models.Model):
  
      tax_group = models.ForeignKey(TaxGroup, related_name='products',
                                    null=True)
  
new_contents: from django.db import models
  from django.utils.translation import ugettext_lazy as _
  
  class TaxGroup(models.Model):
  
      name = models.CharField(_("group name"), max_length=100)
      rate = models.DecimalField(_("rate"), max_digits=4, decimal_places=2,
                                 help_text=_("Percentile rate of the tax."))
      rate_name = models.CharField(_("name of the rate"), max_length=30,
                                   help_text=_("Name of the rate which will be"
                                               " displayed to the user."))
      def __unicode__(self):
          return self.name
  
  
  class TaxedProductMixin(models.Model):
  
      tax_group = models.ForeignKey(TaxGroup, related_name='products',
                                    null=True)
      class Meta:
          abstract = True
  
subject: Fix - TaxedProductMixin is abstract model