Fix product document serialization

apps/search_indexes/serializers.py

@@ -135,6 +135,9 @@ class ProductEstablishmentDocumentSerializer(serializers.Serializer):
     index_name = serializers.CharField()
     city = AnotherCityDocumentShortSerializer()
 
+    def get_attribute(self, instance):
+        return instance.establishment if instance and instance.establishment else None
+
 
 class AddressDocumentSerializer(serializers.Serializer):
     """Address serializer for ES Document."""