ActiveRecord::Transactions::ClassMethods

Transactions are protective blocks where SQL statements are only permanent if they can all succeed as one atomic action. The classic example is a transfer between two accounts where you can only have a deposit if the withdrawal succeeded and vice versa. Transactions enforce the integrity of the database and guard 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:

  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.

Different <a href="/rails/ActiveRecord">ActiveRecord</a> classes in a single transaction

Though the transaction class method is called on some ActiveRecord class, the objects within the transaction block need not all be instances of that class. In this example a Balance record is transactionally saved even though transaction is called on the Account class:

  Account.transaction do
    balance.save!
    account.save!
  end

Transactions are not distributed across database connections

A transaction acts on a single database connection. If you have multiple class-specific databases, the transaction will not protect interaction among them. One workaround is to begin a transaction on each class whose models you alter:

  Student.transaction do
    Course.transaction do
      course.enroll(student)
      student.units += course.units
    end
  end

This is a poor solution, but full distributed transactions are beyond the scope of Active Record.

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 depends on or you can raise exceptions in the callbacks to rollback.

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. One exception is the ActiveRecord::Rollback exception, which will trigger a ROLLBACK when raised, but not be re-raised by the transaction block.

test "transactions" do
assert_raises ZeroDivisionError do
  User.transaction do
    1/0
  end
end
end

Company.transaction do
user.save
company.save
x = 1/0
rescue
exp = $!
begin
  raise ActiveRecord::Rollback
rescue
end
raise exp
end

ActiveRecord::Base.transaction do
foo = Foo.new
foo.save # succeeds

bar = Bar.new
bar.save # failure, validation problem
end

foo = Foo.new
bar = Bar.new

ActiveRecord::Base.transaction do
foo.save! # succeeds
bar.save! # failure, validation problem
end

foo.persisted? # true (!)

foo.reload # raises ActiveRecord::RecordNotFound