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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 <tt>ActiveRecord::StaleObjectError</tt> exception is thrown if that has occurred
# and the update is ignored.
#
# Check out <tt>ActiveRecord::Locking::Pessimistic</tt> 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.
#
# 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 <tt>ActiveRecord::Base.lock_optimistically = false</tt>.
# To override the name of the +lock_version+ column, set the <tt>locking_column</tt> 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 _update_record(attribute_names = self.attribute_names) #:nodoc:
return super unless locking_enabled?
return 0 if attribute_names.empty?
lock_col = self.class.locking_column
previous_lock_value = send(lock_col).to_i
increment_lock
attribute_names += [lock_col]
attribute_names.uniq!
begin
relation = self.class.unscoped
stmt = relation.where(
relation.table[self.class.primary_key].eq(id).and(
relation.table[lock_col].eq(self.class.quote_value(previous_lock_value, column_for_attribute(lock_col)))
)
).arel.compile_update(
arel_attributes_with_values_for_update(attribute_names),
self.class.primary_key
)
affected_rows = self.class.connection.update stmt
unless affected_rows == 1
raise ActiveRecord::StaleObjectError.new(self, "update")
end
affected_rows
# If something went wrong, revert the version.
rescue Exception
send(lock_col + '=', previous_lock_value)
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?
column_name = self.class.locking_column
column = self.class.columns_hash[column_name]
substitute = self.class.connection.substitute_at(column, relation.bind_values.length)
relation = relation.where(self.class.arel_table[column_name].eq(substitute))
relation.bind_values << [column, self[column_name].to_i]
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)
clear_caches_calculated_from_columns
@locking_column = value.to_s
end
# The version column used for optimistic locking. Defaults to +lock_version+.
def locking_column
reset_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
class LockingType < SimpleDelegator # :nodoc:
def type_cast_from_database(value)
# `nil` *should* be changed to 0
super.to_i
end
def changed?(old_value, *)
# Ensure we save if the default was `nil`
super || old_value == 0
end
def init_with(coder)
__setobj__(coder['subtype'])
end
def encode_with(coder)
coder['subtype'] = __getobj__
end
end
end
end
|
