Module: ActiveRecord::Locking::Pessimistic

### Home Classes Methods

### In Files

- lib/active_record/locking/pessimistic.rb

### Methods

- #lock!

### Files

- README.rdoc

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# ActiveRecord::Locking::Pessimistic

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

### Public Instance Methods

lock!(lock = true) click to toggle source

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

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