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 exception is thrown if that has occurred # and the update is ignored. # # Check out ActiveRecord::Locking::Pessimistic for an alternative. # # == Usage # # Active Record supports optimistic locking if the +lock_version+ field 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 an 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 an 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. # # This locking mechanism will function inside a single Ruby process. To make it work across all # web requests, the recommended approach is to add +lock_version+ as a hidden field to your form. # # This behavior can be turned off by setting ActiveRecord::Base.lock_optimistically = false. # To override the name of the +lock_version+ column, set the locking_column class attribute: # # class Person < ActiveRecord::Base # self.locking_column = :lock_person # end # module Optimistic extend ActiveSupport::Concern included do class_attribute :lock_optimistically, instance_writer: false self.lock_optimistically = true end def locking_enabled? #:nodoc: self.class.locking_enabled? end private def increment_lock lock_col = self.class.locking_column previous_lock_value = send(lock_col).to_i send(lock_col + "=", previous_lock_value + 1) end def _create_record(attribute_names = self.attribute_names, *) if locking_enabled? # We always want to persist the locking version, even if we don't detect # a change from the default, since the database might have no default attribute_names |= [self.class.locking_column] end super end def _update_record(attribute_names = self.attribute_names) return super unless locking_enabled? return 0 if attribute_names.empty? begin lock_col = self.class.locking_column previous_lock_value = read_attribute_before_type_cast(lock_col) increment_lock attribute_names.push(lock_col) relation = self.class.unscoped affected_rows = relation.where( self.class.primary_key => id, lock_col => previous_lock_value ).update_all( attributes_for_update(attribute_names).map do |name| [name, _read_attribute(name)] end.to_h ) unless affected_rows == 1 raise ActiveRecord::StaleObjectError.new(self, "update") end affected_rows # If something went wrong, revert the locking_column value. rescue Exception send(lock_col + "=", previous_lock_value.to_i) raise end end def destroy_row affected_rows = super if locking_enabled? && affected_rows != 1 raise ActiveRecord::StaleObjectError.new(self, "destroy") end affected_rows end def relation_for_destroy relation = super if locking_enabled? locking_column = self.class.locking_column relation = relation.where(locking_column => _read_attribute(locking_column)) end relation end module ClassMethods DEFAULT_LOCKING_COLUMN = "lock_version" # 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 locking_column=(value) reload_schema_from_cache @locking_column = value.to_s end # The version column used for optimistic locking. Defaults to +lock_version+. def locking_column @locking_column = DEFAULT_LOCKING_COLUMN unless defined?(@locking_column) @locking_column end # Reset the column used for optimistic locking back to the +lock_version+ default. def reset_locking_column self.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 private # We need to apply this decorator here, rather than on module inclusion. The closure # created by the matcher would otherwise evaluate for `ActiveRecord::Base`, not the # sub class being decorated. As such, changes to `lock_optimistically`, or # `locking_column` would not be picked up. def inherited(subclass) subclass.class_eval do is_lock_column = ->(name, _) { lock_optimistically && name == locking_column } decorate_matching_attribute_types(is_lock_column, :_optimistic_locking) do |type| LockingType.new(type) end end super end end end # In de/serialize we change `nil` to 0, so that we can allow passing # `nil` values to `lock_version`, and not result in `ActiveRecord::StaleObjectError` # during update record. class LockingType < DelegateClass(Type::Value) # :nodoc: def deserialize(value) super.to_i end def serialize(value) super.to_i end def init_with(coder) __setobj__(coder["subtype"]) end def encode_with(coder) coder["subtype"] = __getobj__ end end end end