| Module | ActiveRecord::Locking::Pessimistic |
| In: |
lib/active_record/locking/pessimistic.rb
|
Locking::Pessimistic provides support for row-level locking using SELECT … FOR UPDATE and other lock types.
Pass :lock => true to ActiveRecord::Base.find to obtain an exclusive lock on the selected rows:
# select * from accounts where id=1 for update Account.find(1, :lock => true)
Pass :lock => ‘some locking clause’ to give 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.find(:first, :conditions => "name = 'shugo'", :lock => true)
yuko = Account.find(:first, :conditions => "name = 'yuko'", :lock => true)
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.find(:all, :conditions => ...)
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
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/8.1/interactive/sql-select.html#SQL-FOR-UPDATE-SHARE