cheat sheets.

class User < ActiveRecord::Base
  named_scope :active, :conditions => {:active => true}
  named_scope :inactive, :conditions => {:active => false}
  named_scope :recent, lambda { { :conditions => ['created_at > ?', 1.week.ago]
  } }
end

# Standard usage
User.active    # same as User.find(:all, :conditions => {:active => true})
User.inactive # same as User.find(:all, :conditions => {:active => false})
User.recent   # same as User.find(:all, :conditions => ['created_at > ?',
1.week.ago])

# They're nest-able too!
User.active.recent
  # same as:
  # User.with_scope(:conditions => {:active => true}) do
  #   User.find(:all, :conditions => ['created_at > ?', 1.week.ago])
  # end

# Passing Arguments
class User < ActiveRecord::Base
  named_scope :registered, lambda { |time_ago| { :conditions => ['created_at >
  ?', time_ago] } }
end

User.registered 7.days.ago # same as User.find(:all, :conditions => ['created_at
> ?', 7.days.ago])

# Named scope extensions

class User < ActiveRecord::Base
  named_scope :inactive, :conditions => {:active => false} do
    def activate
      each { |i| i.update_attribute(:active, true) }
    end
  end
end

# Re-activate all inactive users
User.inactive.activate

# Using a subquery..
named_scope :non_administrative, :conditions => [ "users.id NOT IN (SELECT
admins.user_id from admins)" ]
named_scope :administrative, :conditions => [ "users.id IN (SELECT
admins.user_id from admins)" ]

User.administrative
User.non_administrative

Based on a solution at http://railsforum.com/viewtopic.php?id=24563