Transactions

Database transactions

A database transaction groups a series of changes in such a way that either the database applies all of the changes or it applies none of the changes. The classic example of the need for transactions, which is used in Active Record’s own documentation is transferring money between two bank accounts. The basic logic is straightforward:

account1.deposit(100) 
account2.withdraw(100)

However, we have to be careful. What happens if the deposit succeeds but, for some reason, the withdrawal fails. Perhaps the customer is overdrawn? We’ll have added $100 to the balance in account1 without a corresponding deduction from account2. In effect, we’ll have created $100 out of thin air.

Luckily, transactions come to the rescue. A transaction is something like the Three Musketeers with their motto “All for one and one for all.” Within the scope of a transaction, either every SQL statement succeeds or they all have no effect. To put that another way, if any statement fails, the entire transaction has no effect on the database. ...