require 'thread'
module ActiveRecord
# See ActiveRecord::Transactions::ClassMethods for documentation.
module Transactions
extend ActiveSupport::Concern
class TransactionError < ActiveRecordError # :nodoc:
end
included do
define_callbacks :commit, :rollback, :terminator => "result == false", :scope => [:kind, :name]
end
# = Active Record Transactions
#
# 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.
#
# For example:
#
# 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.
#
# == 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
#
# 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 fully distributed transactions are beyond
# the scope of Active Record.
#
# == +save+ and +destroy+ are automatically wrapped in a transaction
#
# 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 <tt>after_*</tt> callbacks.
#
# 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. See below.
#
# == 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 <tt>ActiveRecord::Rollback</tt> exception, which will trigger
# a ROLLBACK when raised, but not be re-raised by the transaction block.
#
# *Warning*: one should not catch <tt>ActiveRecord::StatementInvalid</tt> exceptions
# inside a transaction block. <tt>ActiveRecord::StatementInvalid</tt> 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 cause 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 an
# <tt>ActiveRecord::StatementInvalid</tt> 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, the following behavior may be surprising:
#
# User.transaction do
# User.create(:username => 'Kotori')
# User.transaction do
# User.create(:username => 'Nemu')
# raise ActiveRecord::Rollback
# end
# end
#
# creates both "Kotori" and "Nemu". Reason is the <tt>ActiveRecord::Rollback</tt>
# exception in the nested block does not issue a ROLLBACK. Since these exceptions
# are captured in transaction blocks, the parent block does not see it and the
# real transaction is committed.
#
# In order to get a ROLLBACK for the nested transaction you may ask for a real
# sub-transaction by passing <tt>:requires_new => true</tt>. If anything goes wrong,
# the database rolls back to the beginning of the sub-transaction without rolling
# back the parent transaction. If we add it to the previous example:
#
# User.transaction do
# User.create(:username => 'Kotori')
# User.transaction(:requires_new => true) do
# User.create(:username => 'Nemu')
# raise ActiveRecord::Rollback
# end
# end
#
# only "Kotori" is created. (This works on MySQL and PostgreSQL, but not on SQLite3.)
#
# 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 on MySQL and PostgreSQL. See
# http://dev.mysql.com/doc/refman/5.0/en/savepoint.html
# for more information about savepoints.
#
# === Callbacks
#
# There are two types of callbacks associated with committing and rolling back transactions:
# +after_commit+ and +after_rollback+.
#
# +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.
#
# === 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
def after_commit(*args, &block)
options = args.last
if options.is_a?(Hash) && options[:on]
options[:if] = Array(options[:if])
options[:if] << "transaction_include_action?(:#{options[:on]})"
end
set_callback(:commit, :after, *args, &block)
end
def after_rollback(*args, &block)
options = args.last
if options.is_a?(Hash) && options[:on]
options[:if] = Array(options[:if])
options[:if] << "transaction_include_action?(:#{options[:on]})"
end
set_callback(:rollback, :after, *args, &block)
end
end
# See ActiveRecord::Transactions::ClassMethods for detailed documentation.
def transaction(options = {}, &block)
self.class.transaction(options, &block)
end
def destroy #:nodoc:
with_transaction_returning_status { super }
end
def save(*) #:nodoc:
rollback_active_record_state! do
with_transaction_returning_status { super }
end
end
def save!(*) #:nodoc:
with_transaction_returning_status { super }
end
# Reset id and @new_record if the transaction rolls back.
def rollback_active_record_state!
remember_transaction_record_state
yield
rescue Exception
restore_transaction_record_state
raise
ensure
clear_transaction_record_state
end
# Call the after_commit callbacks
def committed! #:nodoc:
run_callbacks :commit
ensure
clear_transaction_record_state
end
# Call the after rollback callbacks. The restore_state argument indicates if the record
# state should be rolled back to the beginning or just to the last savepoint.
def rolledback!(force_restore_state = false) #:nodoc:
run_callbacks :rollback
ensure
restore_transaction_record_state(force_restore_state)
end
# Add the record to the current transaction so that the :after_rollback and :after_commit callbacks
# can be called.
def add_to_transaction
if self.class.connection.add_transaction_record(self)
remember_transaction_record_state
end
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
status = nil
self.class.transaction do
add_to_transaction
begin
status = yield
rescue ActiveRecord::Rollback
if defined?(@_start_transaction_state)
@_start_transaction_state[:level] = (@_start_transaction_state[:level] || 0) - 1
end
status = nil
end
raise ActiveRecord::Rollback unless status
end
status
end
protected
# Save the new record state and id of a record so it can be restored later if a transaction fails.
def remember_transaction_record_state #:nodoc:
@_start_transaction_state ||= {}
@_start_transaction_state[:id] = id if has_attribute?(self.class.primary_key)
@_start_transaction_state[:new_record] = @new_record
@_start_transaction_state[:destroyed] = @destroyed
@_start_transaction_state[:level] = (@_start_transaction_state[:level] || 0) + 1
end
# Clear the new record state and id of a record.
def clear_transaction_record_state #:nodoc:
if defined?(@_start_transaction_state)
@_start_transaction_state[:level] = (@_start_transaction_state[:level] || 0) - 1
remove_instance_variable(:@_start_transaction_state) if @_start_transaction_state[:level] < 1
end
end
# Restore the new record state and id of a record that was previously saved by a call to save_record_state.
def restore_transaction_record_state(force = false) #:nodoc:
if defined?(@_start_transaction_state)
@_start_transaction_state[:level] = (@_start_transaction_state[:level] || 0) - 1
if @_start_transaction_state[:level] < 1 || force
restore_state = remove_instance_variable(:@_start_transaction_state)
was_frozen = @attributes.frozen?
@attributes = @attributes.dup if was_frozen
@new_record = restore_state[:new_record]
@destroyed = restore_state[:destroyed]
if restore_state.has_key?(:id)
self.id = restore_state[:id]
else
@attributes.delete(self.class.primary_key)
@attributes_cache.delete(self.class.primary_key)
end
@attributes.freeze if was_frozen
end
end
end
# Determine if a record was created or destroyed in a transaction. State should be one of :new_record or :destroyed.
def transaction_record_state(state) #:nodoc:
@_start_transaction_state[state] if defined?(@_start_transaction_state)
end
# Determine if a transaction included an action for :create, :update, or :destroy. Used in filtering callbacks.
def transaction_include_action?(action) #:nodoc:
case action
when :create
transaction_record_state(:new_record)
when :destroy
destroyed?
when :update
!(transaction_record_state(:new_record) || destroyed?)
end
end
end
end
