From db045dbbf60b53dbe013ef25554fd013baf88134 Mon Sep 17 00:00:00 2001
From: David Heinemeier Hansson <david@loudthinking.com>
Date: Wed, 24 Nov 2004 01:04:44 +0000
Subject: Initial

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@4 5ecf4fe2-1ee6-0310-87b1-e25e094e27de
---
 activerecord/lib/active_record/transactions.rb | 119 +++++++++++++++++++++++++
 1 file changed, 119 insertions(+)
 create mode 100644 activerecord/lib/active_record/transactions.rb

(limited to 'activerecord/lib/active_record/transactions.rb')

diff --git a/activerecord/lib/active_record/transactions.rb b/activerecord/lib/active_record/transactions.rb
new file mode 100644
index 0000000000..d440e74346
--- /dev/null
+++ b/activerecord/lib/active_record/transactions.rb
@@ -0,0 +1,119 @@
+require 'active_record/vendor/simple.rb'
+require 'thread'
+
+module ActiveRecord
+  module Transactions # :nodoc:
+    TRANSACTION_MUTEX = Mutex.new
+
+    def self.append_features(base)
+      super
+      base.extend(ClassMethods)
+
+      base.class_eval do
+        alias_method :destroy_without_transactions, :destroy
+        alias_method :destroy, :destroy_with_transactions
+
+        alias_method :save_without_transactions, :save
+        alias_method :save, :save_with_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 succedded and
+    # vice versa. Transaction enforce the integrity of the database and guards 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.
+    #
+    # == 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.
+    #
+    # == Object-level transactions
+    #
+    # You can enable object-level transactions for Active Record objects, though. You do this by naming the each of the Active Records
+    # that you want to enable object-level transactions for, like this:
+    #
+    #   Account.transaction(david, mary) do
+    #     david.withdrawal(100)
+    #     mary.deposit(100)
+    #   end
+    #
+    # If the transaction fails, David and Mary will be returned to their pre-transactional state. No money will have changed hands in
+    # neither object nor database.
+    #
+    # == 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.
+    #
+    # Tribute: Object-level transactions are implemented by Transaction::Simple by Austin Ziegler.
+    module ClassMethods      
+      def transaction(*objects, &block)
+        TRANSACTION_MUTEX.lock
+
+        begin
+          objects.each { |o| o.extend(Transaction::Simple) }
+          objects.each { |o| o.start_transaction }
+
+          result = connection.transaction(&block)
+
+          objects.each { |o| o.commit_transaction }
+          return result
+        rescue Exception => object_transaction_rollback
+          objects.each { |o| o.abort_transaction }
+          raise
+        ensure
+          TRANSACTION_MUTEX.unlock
+        end
+      end
+    end
+
+    def transaction(*objects, &block)
+      self.class.transaction(*objects, &block)
+    end
+
+    def destroy_with_transactions #:nodoc:
+      if TRANSACTION_MUTEX.locked?
+        destroy_without_transactions
+      else
+        transaction { destroy_without_transactions }
+      end
+    end
+    
+    def save_with_transactions(perform_validation = true) #:nodoc:
+      if TRANSACTION_MUTEX.locked?
+        save_without_transactions(perform_validation)
+      else
+        transaction { save_without_transactions(perform_validation) }
+      end
+    end
+  end
+end
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