Merge pull request #4531 from exviva/pessimistic_with_lock

Add ActiveRecord::Base#with_lock

3 files changed, 70 insertions, 1 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 9551987de1..2f78c812c1 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,5 +1,32 @@
 ## Rails 3.2.0 (unreleased) ##
 
+*   Added 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
+
+    *Olek Janiszewski*
+
 *   'on' and 'ON' boolean columns values are type casted to true
     *Santiago Pastorino*
 
@@ -10,7 +37,7 @@
     Example:
       rake db:migrate SCOPE=blog
 
-   *Piotr Sarnacki*
+    *Piotr Sarnacki*
 
 *   Migrations copied from engines are now scoped with engine's name,
     for example 01_create_posts.blog.rb. *Piotr Sarnacki*
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
diff --git a/activerecord/test/cases/locking_test.rb b/activerecord/test/cases/locking_test.rb
index be7edb858f..6da9830525 100644
--- a/activerecord/test/cases/locking_test.rb
+++ b/activerecord/test/cases/locking_test.rb
@@ -415,6 +415,26 @@ unless current_adapter?(:SybaseAdapter, :OpenBaseAdapter) || in_memory_db?
       end
     end
 
+    def test_with_lock_commits_transaction
+      person = Person.find 1
+      person.with_lock do
+        person.first_name = 'fooman'
+        person.save!
+      end
+      assert_equal 'fooman', person.reload.first_name
+    end
+
+    def test_with_lock_rolls_back_transaction
+      person = Person.find 1
+      old = person.first_name
+      person.with_lock do
+        person.first_name = 'fooman'
+        person.save!
+        raise 'oops'
+      end rescue nil
+      assert_equal old, person.reload.first_name
+    end
+
     if current_adapter?(:PostgreSQLAdapter, :OracleAdapter)
       def test_no_locks_no_wait
         first, second = duel { Person.find 1 }