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Merge pull request #4531 from exviva/pessimistic_with_lock

Add ActiveRecord::Base#with_lock
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2 parents 01cde0b + 7afbc89 commit 0056a7512ae15ca9d43ce8b1dd6600962893be58 @tenderlove tenderlove committed Jan 19, 2012
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29 activerecord/CHANGELOG.md
@@ -72,6 +72,33 @@
## 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*
@@ -82,7 +109,7 @@
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*
View
22 activerecord/lib/active_record/locking/pessimistic.rb
@@ -38,6 +38,18 @@ module Locking
# 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 @@ def lock!(lock = true)
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
View
20 activerecord/test/cases/locking_test.rb
@@ -388,6 +388,26 @@ def test_sane_lock_method
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 }
View
11 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.

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