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, 22 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/locking/pessimistic.rb b/activerecord/lib/active_record/locking/pessimistic.rb
index 66994e4797..58af92f0b1 100644
--- a/activerecord/lib/active_record/locking/pessimistic.rb
+++ b/activerecord/lib/active_record/locking/pessimistic.rb
@@ -38,6 +38,18 @@ module ActiveRecord
     #     account2.save!
     #   end
     #
+    # You can start a transaction and acquire the lock in one go by calling
+    # <tt>with_lock</tt> with a block. The block is called from within
+    # a transaction, the object is already locked. Example:
+    #
+    #   account = Account.first
+    #   account.with_lock do
+    #     # This block is called within a transaction,
+    #     # account is already locked.
+    #     account.balance -= 100
+    #     account.save!
+    #   end
+    #
     # Database-specific information on row locking:
     #   MySQL: http://dev.mysql.com/doc/refman/5.1/en/innodb-locking-reads.html
     #   PostgreSQL: http://www.postgresql.org/docs/current/interactive/sql-select.html#SQL-FOR-UPDATE-SHARE
@@ -50,6 +62,16 @@ module ActiveRecord
         reload(:lock => lock) if persisted?
         self
       end
+
+      # Wraps the passed block in a transaction, locking the object
+      # before yielding. You pass can the SQL locking clause
+      # as argument (see <tt>lock!</tt>).
+      def with_lock(lock = true)
+        transaction do
+          lock!(lock)
+          yield
+        end
+      end
     end
   end
 end