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 73 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 136 def _lock_value_for_database(locking_column) if will_save_change_to_attribute?(locking_column) @attributes[locking_column].value_for_database else @attributes[locking_column].original_value_for_database end end
# File lib/active_record/locking/optimistic.rb, line 82 def _touch_row(attribute_names, time) @_touch_attr_names << self.class.locking_column if locking_enabled? super end
# File lib/active_record/locking/optimistic.rb, line 87 def _update_row(attribute_names, attempted_action = "update") return super unless locking_enabled? begin locking_column = self.class.locking_column lock_attribute_was = @attributes[locking_column] lock_value_for_database = _lock_value_for_database(locking_column) attribute_names = attribute_names.dup if attribute_names.frozen? attribute_names << locking_column self[locking_column] += 1 affected_rows = self.class._update_record( attributes_with_values(attribute_names), @primary_key => id_in_database, locking_column => lock_value_for_database ) 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 @attributes[locking_column] = lock_attribute_was raise end end
# File lib/active_record/locking/optimistic.rb, line 119 def destroy_row return super unless locking_enabled? locking_column = self.class.locking_column affected_rows = self.class._delete_record( @primary_key => id_in_database, locking_column => _lock_value_for_database(locking_column) ) if affected_rows != 1 raise ActiveRecord::StaleObjectError.new(self, "destroy") end affected_rows end