+ids+ can be used to pluck all the IDs for the relation using the table's primary key.
<ruby>
Person.ids
# SELECT id FROM people
</ruby>
<ruby>
class Person < ActiveRecord::Base
self.primary_key = "person_id"
end
Person.ids
# SELECT person_id FROM people
</ruby>
h3. Existence of Objects
If you simply want to check for the existence of the object there's a method called +exists?+. This method will query the database using the same query as +find+, but instead of returning an object or collection of objects it will return either +true+ or +false+.
