You might want to store the unique ID of the coupon (maybe its document ID, or maybe a unique identifier generated by some other means) in a document belonging to the user so that you can simply add or remove coupon IDs to that users document.
For example, you might have a structure like users/{user ID} that has a document storing an array of coupon IDs. When a user uses a coupon, it can fetch that coupons information from coupons/{coupon ID} or shops/{shop ID}/coupons/{coupon ID}, then simply remove the coupon ID from the user’s array, thus invalidating their coupon without invalidating the coupon’s data for any other users with the same coupon.
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