Adds lock/unlock functionality to a Ruby on Rails model using a deleted_at column in the corresponding table. To use it, simply "include Lockable" in your model (Rails 2.3, for Rails 3 change named_scope to scope)

lockable.rb

module Lockable
  def self.included(base)
    base.class_eval do
      named_scope :not_locked, :conditions => {:deleted_at => nil}
      named_scope :locked,     :conditions => "#{table_name}.deleted_at IS NOT NULL"
    end
  end

  # Locks the record by setting the deleted_at attribute
  # to the current time.
  # Triggers the +before_lock+ callback if defined.
  #   If this callback returns false, the record is not locked!
  # Triggers the +after_lock+ callback if it is defined.
  #--------------------------------------------------------------
  def lock
    self.deleted_at = Time.now

    if !self.respond_to?(:before_lock) || self.before_lock
      if self.save(false)
        self.after_lock if self.respond_to?(:after_lock)
      end
    end
  end

  # Unlocks the given record by clearing the deleted_at attribute
  # Notice: When unlocking records, validations are executed
  #         to make sure that no invalid unlocked records exist.
  # Triggers the +after_unlock+ callback if it is defined
  # Triggers the +before_unlock+ callback if it is defined.
  #   If this callback returns false, the record is not unlocked!
  #--------------------------------------------------------------
  def unlock
    self.deleted_at = nil
    if !self.respond_to?(:before_unlock) || self.before_unlock
      if self.save
        self.after_unlock if self.respond_to?(:after_unlock)
      end
    end
  end

  def locked?
    !self.deleted_at.nil?
  end

end