Module: ActiveRecord::Locking::Pessimistic

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

### Methods

lock!

### 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.

[Validate]