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.
ActiveRecord::Base.transaction do david.withdrawal(100) mary.deposit(100) end
This example will only take money from David and give it to Mary if neither withdrawal nor deposit raise an exception. Exceptions will force a ROLLBACK that returns the database to the state before the transaction began. Be aware, though, that the objects will not have their instance data returned to their pre-transactional state.
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. This is because transactions are per-database connection, not per-model.
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
The transaction method is also available as a model instance method. For example, you can also do this:
balance.transaction do balance.save! account.save! end
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 fully distributed transactions are beyond the scope of ActiveRecord.
Both #save and #destroy come wrapped in a transaction that ensures that whatever you do in validations or callbacks will happen under its protected cover. So you can use validations to check for values that the transaction depends on or you can raise exceptions in the callbacks to rollback, including
As a consequence changes to the database are not seen outside your connection until the operation is complete. For example, if you try to update the index of a search engine in
after_save the indexer won't see the updated record. The
after_commit callback is the only one that is triggered once the update is committed.
There are two types of callbacks associated with committing and rolling back transactions:
after_commit callbacks are called on every record saved or destroyed within a transaction immediately after the transaction is committed.
after_rollback callbacks are called on every record saved or destroyed within a transaction immediately after the transaction or savepoint is rolled back.
These callbacks are useful for interacting with other systems since you will be guaranteed that the callback is only executed when the database is in a permanent state. For example,
after_commit is a good spot to put in a hook to clearing a cache since clearing it from within a transaction could trigger the cache to be regenerated before the database is updated.
ActiveRecord::Base.transaction uses the
ActiveRecord::Rollback exception to distinguish a deliberate rollback from other exceptional situations. Normally, raising an exception will cause the
.transaction method to rollback the database transaction and pass on the exception. But if you raise an
ActiveRecord::Rollback exception, then the database transaction will be rolled back, without passing on the exception.
For example, you could do this in your controller to rollback a transaction:
class BooksController < ActionController::Base def create Book.transaction do book = Book.new(params[:book]) book.save! if today_is_friday? # The system must fail on Friday so that our support department # won't be out of job. We silently rollback this transaction # without telling the user. raise ActiveRecord::Rollback, "Call tech support!" end end # ActiveRecord::Rollback is the only exception that won't be passed on # by ActiveRecord::Base.transaction, so this line will still be reached # even on Friday. redirect_to root_url end end