Fetch rows that are not present in another table

[harishannam]

How to write a GQL query for the below SQL query?

SELECT od.order_key FROM order_details od LEFT JOIN redeemed_orders ro ON ro.order_key = od.order_key WHERE ro.order_key IS NULL

This query fetches the expected results but I am not sure on how to fetch data the similar way in GQL query.

I already have an Array relationship redemption_details . order_key  → order_details . order_key added tot he order_details table