gistfile1.rb

class Notification
  property :name, String
  belongs_to :user
end

class User
  has n, :notifications
end

# Find all users with no notifications
User.all(:notifications => nil)

# Find all users with no notifications w/ a particular name?
# This doesn't work as expected, because "notifications.name" will
# be turned into an inner join, making the :notifications => nil condition
# moot.
User.all(:notifications => nil, "notifications.name" => "SomeName")

# Is this the only way to accomplish this?
User.has n, :some_name_notifications, :name => "SomeName"
User.all(:some_name_notifications => nil)

# Totally untested
User.all - User.all('notifications.name' => 'SomeName')
User.all(:notifications => {:name.not => "SomeName")) & User.all(:notification => nil)

This type of "negative" query generally sucks.