old_contents: from django.db import models
  from django.utils import timezone
  
  
  class Currency(models.Model):
      cur_code = models.CharField(max_length=8, primary_key=True)
      shortcut = models.CharField(max_length=1)
      symbol = models.CharField(max_length=24)
      long_name = models.CharField(max_length=128)
  
      class Meta:
          unique_together = [['shortcut'], ['long_name']]
  
      def __str__(self):
          return self.cur_code
  
  
  class Book(models.Model):
      book_id = models.AutoField(primary_key=True)
      name = models.CharField(max_length=64)
      created_at = models.DateTimeField()
  
      def set_created_at(self):
          "Instead of auto_now_add"
          if self.created_at is None:
              self.created_at = timezone.now()
  
      def save(self, *args, **kwargs):
          self.set_created_at()
          super(Book, self).save(*args, **kwargs)
  
      class Meta:
          ordering = ['created_at']
          unique_together = [['name']]
  
  
  class AccountType(models.Model):
      account_type = models.CharField(max_length=128, primary_key=True)
      # (credits increase account balance) <=> (sign == True)
      sign = models.BooleanField(verbose_name="Credits increase balance", default=False)
      credit_term = models.CharField(max_length=32)
      debit_term = models.CharField(max_length=32)
  
      def __str__(self):
          return self.account_type
  
new_contents: from django.db import models
  from django.utils import timezone
  
  
  class Currency(models.Model):
      cur_code = models.CharField(max_length=8, primary_key=True)
      shortcut = models.CharField(max_length=1)
      symbol = models.CharField(max_length=24)
      long_name = models.CharField(max_length=128)
  
      class Meta:
          unique_together = [['shortcut'], ['long_name']]
  
      def __str__(self):
          return self.cur_code
  
  
  class Book(models.Model):
      book_id = models.AutoField(primary_key=True)
      name = models.CharField(max_length=64)
      created_at = models.DateTimeField()
  
      def set_created_at(self):
          "Instead of auto_now_add"
          if self.created_at is None:
              self.created_at = timezone.now()
  
      def save(self, *args, **kwargs):
          self.set_created_at()
          super(Book, self).save(*args, **kwargs)
  
      class Meta:
          ordering = ['created_at']
          unique_together = [['name']]
  
  
  class AccountType(models.Model):
      account_type = models.CharField(max_length=128, primary_key=True)
      # (credits increase account balance) <=> (sign == True)
      sign = models.BooleanField(verbose_name="Credits increase balance",
                                 default=False)
      credit_term = models.CharField(max_length=32)
      debit_term = models.CharField(max_length=32)
  
      def __str__(self):
          return self.account_type
  
subject: Make flake8 happy