131 lines
4.8 KiB
Ruby
131 lines
4.8 KiB
Ruby
require 'thread'
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module ActiveRecord
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module Transactions # :nodoc:
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class TransactionError < ActiveRecordError # :nodoc:
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end
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def self.included(base)
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base.extend(ClassMethods)
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base.class_eval do
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[:destroy, :save, :save!].each do |method|
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alias_method_chain method, :transactions
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end
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end
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end
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# Transactions are protective blocks where SQL statements are only permanent if they can all succeed as one atomic action.
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# The classic example is a transfer between two accounts where you can only have a deposit if the withdrawal succeeded and
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# vice versa. Transactions enforce the integrity of the database and guard the data against program errors or database break-downs.
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# So basically you should use transaction blocks whenever you have a number of statements that must be executed together or
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# not at all. Example:
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#
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# transaction do
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# david.withdrawal(100)
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# mary.deposit(100)
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# end
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#
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# This example will only take money from David and give to Mary if neither +withdrawal+ nor +deposit+ raises an exception.
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# Exceptions will force a ROLLBACK that returns the database to the state before the transaction was begun. Be aware, though,
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# that the objects will _not_ have their instance data returned to their pre-transactional state.
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#
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# == Different Active Record classes in a single transaction
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#
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# Though the transaction class method is called on some Active Record class,
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# the objects within the transaction block need not all be instances of
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# that class.
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# In this example a <tt>Balance</tt> record is transactionally saved even
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# though <tt>transaction</tt> is called on the <tt>Account</tt> class:
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#
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# Account.transaction do
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# balance.save!
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# account.save!
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# end
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#
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# == Transactions are not distributed across database connections
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#
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# A transaction acts on a single database connection. If you have
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# multiple class-specific databases, the transaction will not protect
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# interaction among them. One workaround is to begin a transaction
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# on each class whose models you alter:
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#
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# Student.transaction do
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# Course.transaction do
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# course.enroll(student)
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# student.units += course.units
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# end
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# end
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#
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# This is a poor solution, but full distributed transactions are beyond
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# the scope of Active Record.
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#
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# == Save and destroy are automatically wrapped in a transaction
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#
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# Both Base#save and Base#destroy come wrapped in a transaction that ensures that whatever you do in validations or callbacks
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# will happen under the protected cover of a transaction. So you can use validations to check for values that the transaction
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# depends on or you can raise exceptions in the callbacks to rollback.
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#
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# == Exception handling
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#
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# Also have in mind that exceptions thrown within a transaction block will be propagated (after triggering the ROLLBACK), so you
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# should be ready to catch those in your application code. One exception is the ActiveRecord::Rollback exception, which will
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# trigger a ROLLBACK when raised, but not be re-raised by the transaction block.
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module ClassMethods
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def transaction(&block)
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increment_open_transactions
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begin
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connection.transaction(Thread.current['start_db_transaction'], &block)
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ensure
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decrement_open_transactions
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end
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end
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private
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def increment_open_transactions #:nodoc:
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open = Thread.current['open_transactions'] ||= 0
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Thread.current['start_db_transaction'] = open.zero?
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Thread.current['open_transactions'] = open + 1
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end
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def decrement_open_transactions #:nodoc:
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Thread.current['open_transactions'] -= 1
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end
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end
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def transaction(&block)
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self.class.transaction(&block)
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end
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def destroy_with_transactions #:nodoc:
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transaction { destroy_without_transactions }
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end
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def save_with_transactions(perform_validation = true) #:nodoc:
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rollback_active_record_state! { transaction { save_without_transactions(perform_validation) } }
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end
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def save_with_transactions! #:nodoc:
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rollback_active_record_state! { transaction { save_without_transactions! } }
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end
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# Reset id and @new_record if the transaction rolls back.
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def rollback_active_record_state!
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id_present = has_attribute?(self.class.primary_key)
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previous_id = id
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previous_new_record = new_record?
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yield
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rescue Exception
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@new_record = previous_new_record
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if id_present
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self.id = previous_id
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else
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@attributes.delete(self.class.primary_key)
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@attributes_cache.delete(self.class.primary_key)
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end
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raise
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end
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end
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end
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