old_contents: class HQAccountingSetupError(Exception):
      pass
  
new_contents: from django.conf import settings
  from django.core.exceptions import ObjectDoesNotExist
  from django.db import models
  
  
  class HQAccountingSetupError(Exception):
      pass
  
  
  class Currency(models.Model):
      """
      Keeps track of the current conversion rates so that we don't have to poll the free, but rate limited API
      from Open Exchange Rates. Necessary for billing things like MACH SMS.
      """
      code = models.CharField(max_length=3, unique=True)
      name = models.CharField(max_length=25, db_index=True)
      symbol = models.CharField(max_length=10)
      rate_to_usd = models.DecimalField(default=1.0, max_digits=10, decimal_places=9)
      date_updated = models.DateField(auto_now=True)
  
      @classmethod
      def get_default(cls):
          try:
              return cls.objects.get(code=settings.DEFAULT_CURRENCY)
          except ObjectDoesNotExist:
              raise HQAccountingSetupError("You need initialize a default currency. Your current default is set to: %s"
                                           % settings.DEFAULT_CURRENCY)
  
subject: add currency to accounting models