Order.php

<?php
class Order extends ActiveRecord\Model
{
	// order belongs to a person
	static $belongs_to = array(
		array('person'));

	// order can have many payments by many people
	// the conditions is just there as an example as it makes no logical sense
	static $has_many = array(
		array('payments'),
		array('people',
			'through'    => 'payments',
			'select'     => 'people.*, payments.amount',
			'conditions' => 'payments.amount < 200'));

	// order must have a price and tax > 0
	static $validates_numericality_of = array(
		array('price', 'greater_than' => 0),
		array('tax',   'greater_than' => 0));

	// setup a callback to automatically apply a tax
	static $before_validation_on_create = array('apply_tax');

	public function apply_tax()
	{
		if ($this->person->state == 'VA')
			$tax = 0.045;
		elseif ($this->person->state == 'CA')
			$tax = 0.10;
		else
			$tax = 0.02;

		$this->tax = $this->price * $tax;
	}
}