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require 'thread'
module ActiveRecord
module Transactions # :nodoc:
class TransactionError < ActiveRecordError # :nodoc:
end
def self.included(base)
base.extend(ClassMethods)
base.class_eval do
[:destroy, :save, :save!].each do |method|
alias_method_chain method, :transactions
end
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:
#
# 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.
#
# == Rolling back a transaction manually
#
# Instead of relying on exceptions to rollback your transactions, you can also do so manually from within the scope
# of the transaction by accepting a yield parameter and calling rollback! on it. Example:
#
# transaction do |transaction|
# david.withdrawal(100)
# mary.deposit(100)
# transaction.rollback! # rolls back the transaction that was otherwise going to be successful
# 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
# depend 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.
module ClassMethods
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
private
def increment_open_transactions #:nodoc:
open = Thread.current['open_transactions'] ||= 0
Thread.current['start_db_transaction'] = open.zero?
Thread.current['open_transactions'] = open + 1
end
def decrement_open_transactions #:nodoc:
Thread.current['open_transactions'] -= 1
end
end
def transaction(&block)
self.class.transaction(&block)
end
def destroy_with_transactions #:nodoc:
transaction { destroy_without_transactions }
end
def save_with_transactions(perform_validation = true) #:nodoc:
rollback_active_record_state! { transaction { save_without_transactions(perform_validation) } }
end
def save_with_transactions! #:nodoc:
rollback_active_record_state! { 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)
end
raise
end
end
end
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