module ActiveRecord module Locking # Locking::Pessimistic provides support for row-level locking using # SELECT ... FOR UPDATE and other lock types. # # Chain ActiveRecord::Base#find to ActiveRecord::QueryMethods#lock to obtain an exclusive # lock on the selected rows: # # select * from accounts where id=1 for update # Account.lock.find(1) # # Call lock('some locking clause') to use a database-specific locking clause # of your own such as 'LOCK IN SHARE MODE' or 'FOR UPDATE NOWAIT'. Example: # # Account.transaction do # # select * from accounts where name = 'shugo' limit 1 for update # shugo = Account.where("name = 'shugo'").lock(true).first # yuko = Account.where("name = 'yuko'").lock(true).first # shugo.balance -= 100 # shugo.save! # yuko.balance += 100 # yuko.save! # end # # You can also use ActiveRecord::Base#lock! method to lock one record by id. # This may be better if you don't need to lock every row. Example: # # Account.transaction do # # select * from accounts where ... # accounts = Account.where(...) # account1 = accounts.detect { |account| ... } # account2 = accounts.detect { |account| ... } # # select * from accounts where id=? for update # account1.lock! # account2.lock! # account1.balance -= 100 # account1.save! # account2.balance += 100 # account2.save! # end # # You can start a transaction and acquire the lock in one go by calling # with_lock 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/5.7/en/innodb-locking-reads.html # PostgreSQL: http://www.postgresql.org/docs/current/interactive/sql-select.html#SQL-FOR-UPDATE-SHARE module Pessimistic # Obtain a row lock on this record. Reloads the record to obtain the requested # lock. Pass an SQL locking clause to append the end of the SELECT statement # or pass true for "FOR UPDATE" (the default, an exclusive row lock). Returns # the locked record. def lock!(lock = true) reload(:lock => lock) if persisted? self end # Wraps the passed block in a transaction, locking the object # before yielding. You can pass the SQL locking clause # as argument (see lock!). def with_lock(lock = true) transaction do lock!(lock) yield end end end end end