Add ActiveRecord::Base#with_lock

Add a `with_lock` method to ActiveRecord objects, which starts
a transaction, locks the object (pessimistically) and yields to the block.
The method takes one (optional) parameter and passes it to `lock!`.

Before:

    class Order < ActiveRecord::Base
      def cancel!
        transaction do
          lock!
          # ... cancelling logic
        end
      end
    end

After:

    class Order < ActiveRecord::Base
      def cancel!
        with_lock do
          # ... cancelling logic
        end
      end
    end

1 files changed, 11 insertions, 0 deletions

diff --git a/railties/guides/source/active_record_querying.textile b/railties/guides/source/active_record_querying.textile
index beada85ce3..5970a45839 100644
--- a/railties/guides/source/active_record_querying.textile
+++ b/railties/guides/source/active_record_querying.textile
@@ -692,6 +692,17 @@ Item.transaction do
 end
 </ruby>
 
+If you already have an instance of your model, you can start a transaction and acquire the lock in one go using the following code:
+
+<ruby>
+item = Item.first
+item.with_lock do
+  # This block is called within a transaction,
+  # item is already locked.
+  item.increment!(:views)
+end
+</ruby>
+
 h3. Joining Tables
 
 Active Record provides a finder method called +joins+ for specifying +JOIN+ clauses on the resulting SQL. There are multiple ways to use the +joins+ method.