module ActiveRecord::Locking::Optimistic
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
Private Instance Methods
# File lib/active_record/locking/optimistic.rb, line 63 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
# File lib/active_record/locking/optimistic.rb, line 72 def _touch_row(attribute_names, time) super ensure clear_attribute_change(self.class.locking_column) if locking_enabled? end
# File lib/active_record/locking/optimistic.rb, line 78 def _update_row(attribute_names, attempted_action = "update") return super unless locking_enabled? begin locking_column = self.class.locking_column previous_lock_value = read_attribute_before_type_cast(locking_column) attribute_names << locking_column self[locking_column] += 1 affected_rows = self.class._update_record( attributes_with_values(attribute_names), self.class.primary_key => id_in_database, locking_column => previous_lock_value ) if affected_rows != 1 raise ActiveRecord::StaleObjectError.new(self, attempted_action) end affected_rows # If something went wrong, revert the locking_column value. rescue Exception self[locking_column] = previous_lock_value.to_i raise end end
# File lib/active_record/locking/optimistic.rb, line 107 def destroy_row return super unless locking_enabled? locking_column = self.class.locking_column affected_rows = self.class._delete_record( self.class.primary_key => id_in_database, locking_column => read_attribute_before_type_cast(locking_column) ) if affected_rows != 1 raise ActiveRecord::StaleObjectError.new(self, "destroy") end affected_rows end