Module: ActiveRecord::Transactions::ClassMethods

Defined in:
lib/active_record/transactions.rb

Overview

Transactions are protective blocks where SQL statements are only permanent if they can all succed as one atomic action. The classic example is a transfer between two accounts where you can only have a deposit if the withdrawal succedded and vice versa. Transaction enforce the integrity of the database and guards the data against program errors or database break-downs. So basically you should use transaction blocks whenever you have a number of statements that must be executed together or not at all. Example:

Account.transaction do
  david.withdrawal(100)
  mary.deposit(100)
end

This example will only take money from David and give to Mary if neither withdrawal nor deposit raises an exception. Exceptions will force a ROLLBACK that returns the database to the state before the transaction was begun. Be aware, though, that the objects by default will not have their instance data returned to their pre-transactional state.

Save and destroy are automatically wrapped in a transaction

Both Base#save and Base#destroy come wrapped in a transaction that ensures that whatever you do in validations or callbacks will happen under the protected cover of a transaction. So you can use validations to check for values that the transaction depend on or you can raise exceptions in the callbacks to rollback.

Object-level transactions

You can enable object-level transactions for Active Record objects, though. You do this by naming the each of the Active Records that you want to enable object-level transactions for, like this:

Account.transaction(david, mary) do
  david.withdrawal(100)
  mary.deposit(100)
end

If the transaction fails, David and Mary will be returned to their pre-transactional state. No money will have changed hands in neither object nor database.

Exception handling

Also have in mind that exceptions thrown within a transaction block will be propagated (after triggering the ROLLBACK), so you should be ready to catch those in your application code.

Tribute: Object-level transactions are implemented by Transaction::Simple by Austin Ziegler.

Instance Method Summary collapse

Instance Method Details

#transaction(*objects, &block) ⇒ Object 



62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
 
# File 'lib/active_record/transactions.rb', line 62

def transaction(*objects, &block)
  TRANSACTION_MUTEX.lock

  begin
    objects.each { |o| o.extend(Transaction::Simple) }
    objects.each { |o| o.start_transaction }
    connection.begin_db_transaction

    block.call
  
    connection.commit_db_transaction
    objects.each { |o| o.commit_transaction }
  rescue Exception => exception
    connection.rollback_db_transaction
    objects.each { |o| o.abort_transaction }
    raise exception
  ensure
    TRANSACTION_MUTEX.unlock
  end
end