Add support for primary address in user.

Closes #35

Fixes #35

uncloud_auth/serializers.py

@@ -2,15 +2,24 @@ from django.contrib.auth import get_user_model
 from rest_framework import serializers
 
 from uncloud_pay import AMOUNT_DECIMALS, AMOUNT_MAX_DIGITS
+from uncloud_pay.models import BillingAddress
 
 class UserSerializer(serializers.ModelSerializer):
-
-    balance = serializers.DecimalField(max_digits=AMOUNT_MAX_DIGITS,
-                                       decimal_places=AMOUNT_DECIMALS)
-
     class Meta:
         model = get_user_model()
-        fields = ['username', 'email', 'balance', 'maximum_credit' ]
+        read_only_fields = [ 'username', 'balance', 'maximum_credit' ]
+        fields =  read_only_fields + [ 'email', 'primary_billing_address' ]
+
+    def validate(self, data):
+        """
+        Ensure that the primary billing address belongs to the user
+        """
+
+        if 'primary_billing_address' in data:
+            if not data['primary_billing_address'].owner == self.instance:
+                raise serializers.ValidationError("Invalid data")
+
+        return data
 
 class ImportUserSerializer(serializers.Serializer):
     username = serializers.CharField()