Leon, I think a left outer join of Users on Declarations would do it.
in SQL that would be: SELECT * from Users left outer join Declarations ON Users.user_id = Declarations.user_id WHERE Declarations.user_id IS NULL
in ActiveRecord, an :includes does a left outer join so (I may not get this 100% right) User.find(:all, :conditions => "declarations.user_id is NULL", :include => "declarations")
Cheers, --Kip