module ActiveRecord::Locking::Optimistic - activerecord-4.2.3 Documentation

# 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 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 `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

destroy_row() click to toggle source
Calls superclass method

    # File lib/active_record/locking/optimistic.rb, line 114
    def destroy_row
      affected_rows = super

      if locking_enabled? && affected_rows != 1
        raise ActiveRecord::StaleObjectError.new(self, "destroy")
      end

      affected_rows
    end

increment_lock() click to toggle source

    # File lib/active_record/locking/optimistic.rb, line 63
    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

relation_for_destroy() click to toggle source
Calls superclass method

    # File lib/active_record/locking/optimistic.rb, line 124
    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 = relation.where(self.class.arel_table[column_name].eq(substitute))
        relation.bind_values << [column, self[column_name].to_i]
      end

      relation
    end