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

_create_record(attribute_names = self.attribute_names) click to toggle source
Calls superclass method
# 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
_lock_value_for_database(locking_column) click to toggle source
# 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
_touch_row(attribute_names, time) click to toggle source
Calls superclass method
# 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
_update_row(attribute_names, attempted_action = "update") click to toggle source
Calls superclass method
# 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
destroy_row() click to toggle source
Calls superclass method
# 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