require 'thread' module ActiveRecord # See ActiveRecord::Transactions::ClassMethods for documentation. module Transactions extend ActiveSupport::DependencyModule class TransactionError < ActiveRecordError # :nodoc: end included do [:destroy, :save, :save!].each do |method| alias_method_chain method, :transactions end end # 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: # # ActiveRecord::Base.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 will _not_ have their instance # data returned to their pre-transactional state. # # == Different Active Record classes in a single transaction # # Though the transaction class method is called on some Active Record 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 # # Note that 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 # # == 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, including after_* callbacks. # # == Exception handling and rolling back # # 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. # # *Warning*: one should not catch ActiveRecord::StatementInvalid exceptions # inside a transaction block. StatementInvalid exceptions indicate that an # error occurred at the database level, for example when a unique constraint # is violated. On some database systems, such as PostgreSQL, database errors # inside a transaction causes the entire transaction to become unusable # until it's restarted from the beginning. Here is an example which # demonstrates the problem: # # # Suppose that we have a Number model with a unique column called 'i'. # Number.transaction do # Number.create(:i => 0) # begin # # This will raise a unique constraint error... # Number.create(:i => 0) # rescue ActiveRecord::StatementInvalid # # ...which we ignore. # end # # # On PostgreSQL, the transaction is now unusable. The following # # statement will cause a PostgreSQL error, even though the unique # # constraint is no longer violated: # Number.create(:i => 1) # # => "PGError: ERROR: current transaction is aborted, commands # # ignored until end of transaction block" # end # # One should restart the entire transaction if a StatementError occurred. # # == Nested transactions # # #transaction calls can be nested. By default, this makes all database # statements in the nested transaction block become part of the parent # transaction. For example: # # User.transaction do # User.create(:username => 'Kotori') # User.transaction do # User.create(:username => 'Nemu') # raise ActiveRecord::Rollback # end # end # # User.find(:all) # => empty # # It is also possible to requires a sub-transaction by passing # :requires_new => true. If anything goes wrong, the # database rolls back to the beginning of the sub-transaction # without rolling back the parent transaction. For example: # # User.transaction do # User.create(:username => 'Kotori') # User.transaction(:requires_new => true) do # User.create(:username => 'Nemu') # raise ActiveRecord::Rollback # end # end # # User.find(:all) # => Returns only Kotori # # Most databases don't support true nested transactions. At the time of # writing, the only database that we're aware of that supports true nested # transactions, is MS-SQL. Because of this, Active Record emulates nested # transactions by using savepoints. See # http://dev.mysql.com/doc/refman/5.0/en/savepoints.html # for more information about savepoints. # # === Caveats # # If you're on MySQL, then do not use DDL operations in nested transactions # blocks that are emulated with savepoints. That is, do not execute statements # like 'CREATE TABLE' inside such blocks. This is because MySQL automatically # releases all savepoints upon executing a DDL operation. When #transaction # is finished and tries to release the savepoint it created earlier, a # database error will occur because the savepoint has already been # automatically released. The following example demonstrates the problem: # # Model.connection.transaction do # BEGIN # Model.connection.transaction(:requires_new => true) do # CREATE SAVEPOINT active_record_1 # Model.connection.create_table(...) # active_record_1 now automatically released # end # RELEASE savepoint active_record_1 # # ^^^^ BOOM! database error! # end # # Note that "TRUNCATE" is also a MySQL DDL statement! module ClassMethods # See ActiveRecord::Transactions::ClassMethods for detailed documentation. def transaction(options = {}, &block) # See the ConnectionAdapters::DatabaseStatements#transaction API docs. connection.transaction(options, &block) end end # See ActiveRecord::Transactions::ClassMethods for detailed documentation. def transaction(&block) self.class.transaction(&block) end def destroy_with_transactions #:nodoc: with_transaction_returning_status(:destroy_without_transactions) end def save_with_transactions(perform_validation = true) #:nodoc: rollback_active_record_state! { with_transaction_returning_status(:save_without_transactions, perform_validation) } end def save_with_transactions! #:nodoc: rollback_active_record_state! { self.class.transaction { save_without_transactions! } } end # Reset id and @new_record if the transaction rolls back. def rollback_active_record_state! id_present = has_attribute?(self.class.primary_key) previous_id = id previous_new_record = new_record? yield rescue Exception @new_record = previous_new_record if id_present self.id = previous_id else @attributes.delete(self.class.primary_key) @attributes_cache.delete(self.class.primary_key) end raise end # Executes +method+ within a transaction and captures its return value as a # status flag. If the status is true the transaction is committed, otherwise # a ROLLBACK is issued. In any case the status flag is returned. # # This method is available within the context of an ActiveRecord::Base # instance. def with_transaction_returning_status(method, *args) status = nil self.class.transaction do status = send(method, *args) raise ActiveRecord::Rollback unless status end status end end end