update special_discount_link method to return a URL based on user authentication

backend/product/serializers.py

@@ -88,7 +88,11 @@ class ProductVariantSerialzier(serializers.ModelSerializer):
             return None
     
     def get_special_discount_link(self, obj):
-        return 'https://'
+        request = self.context.get('request')
+        if request.user.is_authenticated:
+            return 'https://google.com'
+        else:
+            return None
 
     def get_price_after_discount(self, obj):
         return f'{obj.price_after_discount:,.0f} تومانءءء'