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’.


  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
    yuko.balance += 100

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.balance -= 100
    account2.balance += 100

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
Public Instance methods
lock!(lock = true)

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.

    # File activerecord/lib/active_record/locking/pessimistic.rb, line 49
49:       def lock!(lock = true)
50:         reload(:lock => lock) unless new_record?
51:         self
52:       end