saveAll() with return ID

saveAll() with return ID

I have a form where people sign up/make a payment all together.
I'm using saveAll() to add both a row to the payments table, and a row to
the users table. The only problem is, my user_id isn't getting saved along
with the payment data.
Whats the best way to get the newly created user_id and save it with the
payment data?