Model Design Question

Hey all. I'm fairly new to Rails (and ORM for that matter) and have a design question for you. I'm tasked with designing an app to do the following:

* System should be able to handle different types of "tests" * Each test has one more more Samples associated to it * The type of Test will drive what Samples are shown for user input

I've started down the "single table inheritance" path for the Test model:

class Test < AR::Base   has_many :samples end

class TestA < Test   #business logic for TestA end

class TestB < Test   #business logic for TestB end

For argument's sake, lets say that a TestA has 2 samples. When a user selects a TestA on a view, I need to nuke up a TestA for creation. Since the business rule is to force the TestA to have 2 samples, when should I do this in the model? Override the constructor for TestA and call self.samples.add() twice? I really only want 1 complex form to handle the creation of a new test and it's samples.

Thanks for the help!

Geoff