module ActiveRecord module Locking # == What is Optimistic Locking # # Optimistic locking allows multiple users to access the same record for edits, and assumes a minimum of # conflicts with the data. It does this by checking whether another process has made changes to a record since # it was opened, an ActiveRecord::StaleObjectError is thrown if that has occurred and the update is ignored. # # Check out ActiveRecord::Locking::Pessimistic for an alternative. # # == Usage # # 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 # # Optimistic locking will also check for stale data when objects are destroyed. Example: # # p1 = Person.find(1) # p2 = Person.find(1) # # p1.first_name = "Michael" # p1.save # # p2.destroy # 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. # To override the name of the lock_version column, invoke the set_locking_column method. # This method uses the same syntax as set_table_name module Optimistic extend ActiveSupport::Concern included do cattr_accessor :lock_optimistically, :instance_writer => false self.lock_optimistically = true class << self alias_method :locking_column=, :set_locking_column end end def locking_enabled? #:nodoc: self.class.locking_enabled? end private def attributes_from_column_definition result = super # If the locking column has no default value set, # start the lock version at zero. Note we can't use # locking_enabled? at this point as @attributes may # not have been initialized yet if lock_optimistically && result.include?(self.class.locking_column) result[self.class.locking_column] ||= 0 end return result end def update(attribute_names = @attributes.keys) #:nodoc: return super unless locking_enabled? return 0 if attribute_names.empty? lock_col = self.class.locking_column previous_value = send(lock_col).to_i send(lock_col + '=', previous_value + 1) attribute_names += [lock_col] attribute_names.uniq! begin relation = self.class.unscoped affected_rows = relation.where( relation.table[self.class.primary_key].eq(quoted_id).and( relation.table[self.class.locking_column].eq(quote_value(previous_value)) ) ).arel.update(arel_attributes_values(false, false, attribute_names)) unless affected_rows == 1 raise ActiveRecord::StaleObjectError, "Attempted to update a stale object: #{self.class.name}" end affected_rows # If something went wrong, revert the version. rescue Exception send(lock_col + '=', previous_value) raise end end def destroy #:nodoc: return super unless locking_enabled? unless new_record? lock_col = self.class.locking_column previous_value = send(lock_col).to_i table = self.class.arel_table predicate = table[self.class.primary_key].eq(id) predicate = predicate.and(table[self.class.locking_column].eq(previous_value)) affected_rows = self.class.unscoped.where(predicate).delete_all unless affected_rows == 1 raise ActiveRecord::StaleObjectError, "Attempted to delete a stale object: #{self.class.name}" end end @destroyed = true freeze end module ClassMethods DEFAULT_LOCKING_COLUMN = 'lock_version' # Is optimistic locking enabled for this table? Returns true if the # +lock_optimistically+ flag is set to true (which it is, by default) # and the table includes the +locking_column+ column (defaults to # +lock_version+). def locking_enabled? lock_optimistically && columns_hash[locking_column] end # Set the column to use for optimistic locking. Defaults to +lock_version+. def set_locking_column(value = nil, &block) define_attr_method :locking_column, value, &block value end # The version column used for optimistic locking. Defaults to +lock_version+. def locking_column reset_locking_column end # Quote the column name used for optimistic locking. def quoted_locking_column connection.quote_column_name(locking_column) end # Reset the column used for optimistic locking back to the +lock_version+ default. def reset_locking_column set_locking_column DEFAULT_LOCKING_COLUMN end # Make sure the lock version column gets updated when counters are # updated. def update_counters(id, counters) counters = counters.merge(locking_column => 1) if locking_enabled? super end end end end end