Fix python errors on latest hack commits => make runserver happy again

uncloud/uncloud_pay/models.py

@@ -1,8 +1,11 @@
 from django.db import models
 from django.contrib.auth import get_user_model
+from django.core.validators import MinValueValidator
 
-# Create your models here.
+import uuid
 
+AMOUNT_MAX_DIGITS=10
+AMOUNT_DECIMALS=2
 
 class Bill(models.Model):
     owner = models.ForeignKey(get_user_model(),
@@ -35,7 +38,6 @@ class Order(models.Model):
                                        null=True)
 
     bill = models.ManyToManyField(Bill,
-                                  on_delete=models.CASCADE,
                                   editable=False,
                                   blank=True,
                                   null=True)
@@ -77,6 +79,8 @@ class Payment(models.Model):
 
     amount = models.DecimalField(
             default=0.0,
+            max_digits=AMOUNT_MAX_DIGITS,
+            decimal_places=AMOUNT_DECIMALS,
             validators=[MinValueValidator(0)])
 
     source = models.CharField(max_length=256,