Myujth

I have two models, one referring the other one:

class RatesTable(models.Model):
 hotel = models.ForeignKey(Hotel, on_delete=models.PROTECT, related_name='rates_tables', verbose_name=_('Hotel'))
 contract = models.ForeignKey(Contract, on_delete=models.PROTECT, null=True, blank=True, related_name='rates_tables', verbose_name=_('Contract'))
 name = models.CharField(max_length=100, verbose_name=_('Name'))
 description = models.TextField(null=True, blank=True, verbose_name=_('Description'))
 is_base = models.BooleanField(verbose_name=_('Is a base rate-table?'))
 start_date = models.DateField(null=True, blank=True, verbose_name=_('Start Date'))
 due_date = models.DateField(null=True, blank=True, verbose_name=_('Due Date'))
 release = models.IntegerField(verbose_name=_('Release'))

 class Meta:
 verbose_name = _('Rates Table')
 verbose_name_plural = _('Rates Tables')
 unique_together = ('contract', 'start_date', 'due_date')
 indexes = [
 models.Index(fields=['hotel',]),
 ]

 def __str__(self):
 return "[][] -".format(self.id, self.hotel.name, self.contract.id, self.name, self.start_date, self.due_date)

class Rate(models.Model):
 rates_table = models.ForeignKey(RatesTable, on_delete=models.PROTECT, related_name='rates', verbose_name=_('Rates Table'))
 product = models.ForeignKey(Product, on_delete=models.PROTECT, related_name='product_rates', verbose_name=_('Product'))
 rate = models.FloatField(verbose_name=_('Rate'))

 class Meta:
 verbose_name = _('Rate')
 verbose_name_plural = _('Rates')
 unique_together = ('rates_table', 'product')

 def __str__(self):
 return "[][][]".format(self.id, self.rates_table.contract, self.rates_table.id, self.product.name)

Each one has its own standard end-point in urls.py. End-point for model RatesTable works perfectly, but end-point for model Rate raises this error: return "[][] -".format(self.id, self.hotel.name, self.contract.id, self.name, self.start_date, self.due_date)
AttributeError: 'NoneType' object has no attribute 'id', and this error is raised on method __str__(self) of model RatesTable, not model Rate!

I don't know why DRF raises an error in the referenced model. It's the only case it happens, and I have many models with foreign keys