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 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 ActiveRecord 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.
Instance Method Summary collapse
Instance Method Details
#transaction(&block) ⇒ Object
75 76 77 78 79 80 81 82 83 84 85 |
# File 'lib/active_record/transactions.rb', line 75 def transaction(&block) previous_handler = trap('TERM') { raise TransactionError, "Transaction aborted" } increment_open_transactions begin connection.transaction(Thread.current['start_db_transaction'], &block) ensure decrement_open_transactions trap('TERM', previous_handler) end end |