I need to generate a unique serial number in numerical order, but only 
unique for each user.

For example, each user can create a new credit card and each new credit card 
must start at a predetermined number and increment by 1 for each new credit 
card for that user. Thus, credit card numbers are unique for a user but not 
unique for the CreditCard table. And yes, it is absolutely critical that 
credit card numbers are always unique for each user.

http://apidock.com/rails/ActiveRecord/Locking/Pessimistic

I'm using PostgreSQL.

Looking through the docs it doesn't appear that I can use either pessimistic 
locking or optimistic locking. Maybe I'm wrong?

Is my only option to lock the table, find the highest credit card number, 
add 1, update the new credit card number, save, and release the table lock?

Karl


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