has_one

has_one - find all that have no associated object

Let me pose a typical Rails situation: class Person Now, how can you get all the people that don’t have a fancy hat? class Person 'LEFT JOIN fancy_hats ON fancy_hats.person_id = people.id', :conditions = 'fancy_hats.person_id IS NULL' end Now you can find all the hatless people you want. FYI: Finding fancy hats that have no one to wear them is a lot easier, because the foreign key is stored in the fancy_hats table.