Default scope allows you to define a scope applied to every query on a model.

  class User < ActiveRecord::Base
    default_scope :conditions => { :banned => false }
  end

  User.find(:first)
  > 'SELECT * FROM `users` WHERE `users`.`banned` = 0' LIMIT 1;

To execute queries outside the default scope, use with_exclusive_scope

  User.with_exclusive_scope do
    find(:first)
  end