Single Table Infuriation

Single Table Inheritance in Rails is an excellent pattern that works really well, except when it doesn't.

class Parent < ActiveRecord::Base
end

class Child < Parent
end

class GrandChild < Child
end

Each of the above classes are defined in their own files (app/models). There are times in development mode that I've noticed that Child.find(:all) will generate the following SQL:

SELECT * FROM parents WHERE (type='Child')

In Rails 1.2 the fix for this is perhaps a little obfuscated.

class Parent < ActiveRecord::Base
  require_dependency 'child'
end

class Child < Parent
  require_dependency 'grand_child'
end

class GrandChild < Child
end

Now, the other part of this solution is that you probably want to call require_dependency at the end of the class definition as there may be methods in the parent object that the child object needs. I suppose you could re-open the class at the end of the file.