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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
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