some fields validation and model cleaning

payments/models.py

@@ -18,6 +18,11 @@ class Payment(models.Model):
     paid_info = models.TextField(blank=True, null=True)
     description = models.CharField(max_length=255, blank=True, null=True)
 
+    def clean(self):
+        if Payment.objects.filter(invoice_number=self.invoice_number).exclude(id=self.id).exists():
+            raise ValueError(f'Payment with invoice number {self.invoice_number} already exists.')
+
+
 class PaymentItem(models.Model):
     payment = models.ForeignKey(Payment, related_name='payment_items', on_delete=models.CASCADE)
     container = models.ForeignKey(Container, related_name='payment_containers', on_delete=models.CASCADE)
@@ -47,4 +52,4 @@ class AdditionalFees(models.Model):
 
     class Meta:
         verbose_name = 'Additional Fees'
-        verbose_name_plural = 'Additional Fees'
+        verbose_name_plural = 'Additional Fees'