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Locking::Pessimistic provides support for row-level locking using SELECT … FOR UPDATE and other lock types.

Chain ActiveRecord::Base#find to ActiveRecord::QueryMethods#lock to obtain an exclusive lock on the selected rows:

# select * from accounts where id=1 for update
Account.lock.find(1)

Call lock('some locking clause') to use 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 = 'yuko'").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(...)
  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

You can start a transaction and acquire the lock in one go by calling with_lock with a block. The block is called from within a transaction, the object is already locked. Example:

account = Account.first
account.with_lock do
  # This block is called within a transaction,
  # account is already locked.
  account.balance -= 100
  account.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
Methods
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Instance Public 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 61
def lock!(lock = true)
  reload(:lock => lock) if persisted?
  self
end
with_lock(lock = true)

Wraps the passed block in a transaction, locking the object before yielding. You can pass the SQL locking clause as argument (see lock!).

# File activerecord/lib/active_record/locking/pessimistic.rb, line 69
def with_lock(lock = true)
  transaction do
    lock!(lock)
    yield
  end
end