Type:
Module
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!
endYou 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/5.1/en/innodb-locking-reads.html PostgreSQL: http://www.postgresql.org/docs/current/interactive/sql-select.html#SQL-FOR-UPDATE-SHARE