check rancher unique national code in serilaizer validation

apps/herd/exception.py

@@ -0,0 +1,7 @@
+from rest_framework.exceptions import APIException
+from rest_framework import status
+
+
+class UniqueRancherApiException(APIException):
+    status_code = status.HTTP_403_FORBIDDEN
+    default_detail = "دامدار با این کد ملی از قبل در سیستم وجود دارد"  # noqa

apps/herd/pos/api/v1/serializers.py

@@ -4,6 +4,7 @@ from apps.authentication.api.v1.serializers.serializer import (
     ProvinceSerializer,
     CitySerializer
 )
+from apps.herd.exception import UniqueRancherApiException
 from rest_framework import serializers
 from apps.herd.models import Herd, Rancher
 
@@ -35,6 +36,12 @@ class RancherSerializer(serializers.ModelSerializer):
         model = Rancher
         fields = '__all__'
 
+    def validate(self, attrs):
+        """ some validations for operations relate to rancher """
+
+        if self.Meta.model.objects.filter(national_code=attrs['national_code']).exists():
+            raise UniqueRancherApiException()
+
     def to_representation(self, instance):
         """ customize output of serializer """