module ActiveRecord # Active Records support optimistic locking if the field lock_version is present. Each update to the # record increments the lock_version column and the locking facilities ensure that records instantiated twice # will let the last one saved raise a StaleObjectError if the first was also updated. Example: # # p1 = Person.find(1) # p2 = Person.find(1) # # p1.first_name = "Michael" # p1.save # # p2.first_name = "should fail" # p2.save # Raises a ActiveRecord::StaleObjectError # # You're then responsible for dealing with the conflict by rescuing the exception and either rolling back, merging, # or otherwise apply the business logic needed to resolve the conflict. # # You must ensure that your database schema defaults the lock_version column to 0. # # This behavior can be turned off by setting ActiveRecord::Base.lock_optimistically = false. module Locking def self.append_features(base) #:nodoc: super base.class_eval do alias_method :update_without_lock, :update alias_method :update, :update_with_lock end end def update_with_lock #:nodoc: if locking_enabled? previous_value = self.lock_version self.lock_version = previous_value + 1 affected_rows = connection.update(<<-end_sql, "#{self.class.name} Update with optimistic locking") UPDATE #{self.class.table_name} SET #{quoted_comma_pair_list(connection, attributes_with_quotes(false))} WHERE #{self.class.primary_key} = #{quote(id)} AND lock_version = #{quote(previous_value)} end_sql unless affected_rows == 1 raise ActiveRecord::StaleObjectError, "Attempted to update a stale object" end else update_without_lock end end end class Base @@lock_optimistically = true cattr_accessor :lock_optimistically def locking_enabled? #:nodoc: lock_optimistically && respond_to?(:lock_version) end end end