3 files changed, 63 insertions, 2 deletions
diff --git a/activerecord/lib/active_record.rb b/activerecord/lib/active_record.rb
index ecf8499e16..963a14dd3e 100755
--- a/activerecord/lib/active_record.rb
+++ b/activerecord/lib/active_record.rb
@@ -39,11 +39,13 @@ require 'active_record/reflection'
 require 'active_record/timestamp'
 require 'active_record/acts/list'
 require 'active_record/acts/tree'
+require 'active_record/locking'
 
 ActiveRecord::Base.class_eval do
   include ActiveRecord::Validations
-  include ActiveRecord::Timestamp
   include ActiveRecord::Callbacks
+  include ActiveRecord::Locking
+  include ActiveRecord::Timestamp
   include ActiveRecord::Associations
   include ActiveRecord::Aggregations
   include ActiveRecord::Transactions
@@ -55,4 +57,4 @@ end
 require 'active_record/connection_adapters/mysql_adapter'
 require 'active_record/connection_adapters/postgresql_adapter'
 require 'active_record/connection_adapters/sqlite_adapter'
-require 'active_record/connection_adapters/sqlserver_adapter'
-\ No newline at end of file
+require 'active_record/connection_adapters/sqlserver_adapter'
diff --git a/activerecord/lib/active_record/base.rb b/activerecord/lib/active_record/base.rb
index e4283ac157..849fb1578b 100755
--- a/activerecord/lib/active_record/base.rb
+++ b/activerecord/lib/active_record/base.rb
@@ -26,6 +26,8 @@ module ActiveRecord #:nodoc:
   end
   class PreparedStatementInvalid < ActiveRecordError #:nodoc:
   end
+  class StaleObjectError < ActiveRecordError #:nodoc:
+  end
 
   # Active Record objects doesn't specify their attributes directly, but rather infer them from the table definition with
   # which they're linked. Adding, removing, and changing attributes and their type is done directly in the database. Any change
diff --git a/activerecord/lib/active_record/locking.rb b/activerecord/lib/active_record/locking.rb
new file mode 100644
index 0000000000..e08e2fcec8
--- /dev/null
+++ b/activerecord/lib/active_record/locking.rb
@@ -0,0 +1,57 @@
+module ActiveRecord
+  module Locking
+    # Active Records support optimistic locking if the field <tt>lock_version</tt> is present.  Each update to the
+    # record increments the lock_version column and the locking facilities ensure that records instantiated twice
+    # will let the last one saved raise a StaleObjectError if the first was also updated. Example:
+    #
+    #   p1 = Person.find(1)
+    #   p2 = Person.find(1)
+    #   
+    #   p1.first_name = "Michael"
+    #   p1.save
+    #   
+    #   p2.first_name = "should fail"
+    #   p2.save # Raises a ActiveRecord::StaleObjectError
+    #
+    # You're then responsible for dealing with the conflict by rescuing the exception and either rolling back, merging,
+    # or otherwise apply the business logic needed to resolve the conflict.
+    #
+    # You must ensure that your database schema defaults the lock_version column to 0.
+    #
+    # This behavior can be turned off by setting <tt>ActiveRecord::Base.lock_optimistically = false</tt>.
+    def self.append_features(base)
+      super
+      base.class_eval do
+        alias_method :update_without_lock, :update
+        alias_method :update, :update_with_lock
+      end
+    end
+
+    def update_with_lock
+      if locking_enabled?
+        previous_value    = self.lock_version
+        self.lock_version = previous_value + 1
+        
+        affected_rows = connection.update(
+          "UPDATE #{self.class.table_name} "+
+          "SET #{quoted_comma_pair_list(connection, attributes_with_quotes(false))} " +
+          "WHERE #{self.class.primary_key} = #{quote(id)} AND lock_version = #{quote(previous_value)}",
+          "#{self.class.name} Update with optimistic locking"
+        )
+        
+        raise(ActiveRecord::StaleObjectError, "Attempted to update a stale object") unless affected_rows == 1
+      else
+        update_without_lock
+      end
+    end
+  end
+
+  class Base
+    @@lock_optimistically = true
+    cattr_accessor :lock_optimistically
+
+    def locking_enabled?
+      lock_optimistically && respond_to?(:lock_version)
+    end
+  end
+end
+\ No newline at end of file