thorough guide to Locks/Transactions/Isolation Levels in Rails [closed]
I would check out Chapter 5 of The Rails 3 Way for now, particularly Section 5.5 on Database Locking. I would also look at the ActiveRecord::Locking source, ActiveRecord::Transactions source, and the transaction_isolation test case as the code is very readable and if you're going to need some down in the weeds answers, it's there in the code!
I found Differences between transactions and locking on Makandropedia a pretty useful starting point to learn the difference between transactions and locks.