Module: ActiveRecord::Locking::Pessimistic

Defined in:
lib/active_record/locking/pessimistic.rb

Overview

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.where("name = 'shugo'").lock(true).first
  yuko = Account.where("name = 'shugo'").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(...).all
  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/current/interactive/sql-select.html#SQL-FOR-UPDATE-SHARE

Instance Method Summary collapse

Instance Method Details

#lock!(lock = true) ⇒ Object

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.



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# File 'lib/active_record/locking/pessimistic.rb', line 49

def lock!(lock = true)
  reload(:lock => lock) unless new_record?
  self
end