As your business logic gets complex you may need to implement transactions. The classic example is a bank funds transfer from account A to account B. If the withdrawal from account A fails then the deposit to account B should either never take place or be rolled back.
All the complexity is handled by ActiveRecord::Transactions. Any model class or instance has a method named .transaction. When called and passed a block, that block will be executed inside a database transaction. If there's an exception raised, the transaction will automatically be rolled back.