diff --git a/uncloud_pay/migrations/0004_auto_20200801_1604.py b/uncloud_pay/migrations/0004_auto_20200801_1604.py
new file mode 100644
index 0000000..9b87bd6
--- /dev/null
+++ b/uncloud_pay/migrations/0004_auto_20200801_1604.py
@@ -0,0 +1,23 @@
+# Generated by Django 3.0.6 on 2020-08-01 16:04
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('uncloud_pay', '0003_auto_20200621_1442'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='order',
+            name='replaced_by',
+        ),
+        migrations.AddField(
+            model_name='order',
+            name='replaces',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.PROTECT, related_name='replaced_by', to='uncloud_pay.Order'),
+        ),
+    ]
diff --git a/uncloud_pay/migrations/0005_auto_20200801_1626.py b/uncloud_pay/migrations/0005_auto_20200801_1626.py
new file mode 100644
index 0000000..323a4cf
--- /dev/null
+++ b/uncloud_pay/migrations/0005_auto_20200801_1626.py
@@ -0,0 +1,24 @@
+# Generated by Django 3.0.6 on 2020-08-01 16:26
+
+import django.core.validators
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('uncloud_pay', '0004_auto_20200801_1604'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='order',
+            name='one_time_price',
+            field=models.DecimalField(decimal_places=2, default=0.0, max_digits=10, validators=[django.core.validators.MinValueValidator(0)]),
+        ),
+        migrations.AddField(
+            model_name='order',
+            name='recurring_price',
+            field=models.DecimalField(decimal_places=2, default=0.0, max_digits=10, validators=[django.core.validators.MinValueValidator(0)]),
+        ),
+    ]
diff --git a/uncloud_pay/models.py b/uncloud_pay/models.py
index 47c1fe1..844124e 100644
--- a/uncloud_pay/models.py
+++ b/uncloud_pay/models.py
@@ -6,6 +6,9 @@ from django.core.validators import MinValueValidator
 from django.utils import timezone
 from django.core.exceptions import ObjectDoesNotExist, ValidationError
 
+from django.contrib.contenttypes.fields import GenericForeignKey
+from django.contrib.contenttypes.models import ContentType
+
 import logging
 from functools import reduce
 import itertools
@@ -296,13 +299,13 @@ class Order(models.Model):
 
     billing_address = models.ForeignKey(BillingAddress,
                                         on_delete=models.CASCADE)
+
     description = models.TextField()
 
     # TODO: enforce ending_date - starting_date to be larger than recurring_period.
     creation_date = models.DateTimeField(auto_now_add=True)
     starting_date = models.DateTimeField(default=timezone.now)
-    ending_date = models.DateTimeField(blank=True,
-                                       null=True)
+    ending_date = models.DateTimeField(blank=True, null=True)
 
     recurring_period = models.IntegerField(choices = RecurringPeriod.choices,
                                            default = RecurringPeriod.PER_30D)
@@ -317,17 +320,17 @@ class Order(models.Model):
             decimal_places=AMOUNT_DECIMALS,
             validators=[MinValueValidator(0)])
 
-    replaced_by = models.ForeignKey('self',
-                                    related_name='supersede',
-                                    on_delete=models.PROTECT,
-                                    blank=True,
-                                    null=True)
+    replaces = models.ForeignKey('self',
+                                 related_name='replaced_by',
+                                 on_delete=models.PROTECT,
+                                 blank=True,
+                                 null=True)
 
     depends_on = models.ForeignKey('self',
-                                    related_name='parent_of',
-                                    on_delete=models.PROTECT,
-                                    blank=True,
-                                    null=True)
+                                   related_name='parent_of',
+                                   on_delete=models.PROTECT,
+                                   blank=True,
+                                   null=True)
 
     @property
     def count_billed(self):
@@ -395,13 +398,14 @@ class Order(models.Model):
     def records(self):
         return OrderRecord.objects.filter(order=self)
 
-    @property
-    def one_time_price(self):
-        return reduce(lambda acc, record: acc + record.one_time_price, self.records, 0)
+    # these are reald fields!!!
+    # @property
+    # def one_time_price(self):
+    #     return reduce(lambda acc, record: acc + record.one_time_price, self.records, 0)
 
-    @property
-    def recurring_price(self):
-        return reduce(lambda acc, record: acc + record.recurring_price, self.records, 0)
+    # @property
+    # def recurring_price(self):
+    #     return reduce(lambda acc, record: acc + record.recurring_price, self.records, 0)
 
     # Used by uncloud_pay tests.
     @property