Karl Smith wrote in post #978730: > 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.
I hope this "credit card" scenario is a contrived example. It seems to me that it would be critical for the credit card number to be unique across ALL users. > 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? You are correct. Row level locking is useless to prevent the duplication of values between rows due to the race condition during a SQL INSERT. A unique index (possibly across multiple fields) would prevent such duplication. > 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? It might be possible for you to use a sequence. But, in your case you would need a separate sequence for each user if I understand you correctly. Although this might work it may not be appropriate in your case. http://www.postgresql.org/docs/8.1/static/sql-createsequence.html My concern about locking the entire table would be that if something went wrong then you might end up in a state where your entire table is stuck in a locked state. I don't know for sure if that's an issue with PostgreSQL, but something to consider. There may also be other considerations if you ever have a need to use more that one database backend for the purposes of scaling. I would also try to avoid having to use a "max value" query every time you needed to find the next number in sequence. You could instead create a table that contains a foreign key to your users table with the next sequence value stored there. You would then increment that value for each insert similar to how database sequences work. This way you should only have to be concerned about concurrent access to the table used for sequencing. Good luck, I hope this helps at least a little. -- Posted via http://www.ruby-forum.com/. -- You received this message because you are subscribed to the Google Groups "Ruby on Rails: Talk" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/rubyonrails-talk?hl=en.
