gistfile1.php

<?php

//...

class Business extends DataObject {
	//...
	private static $has_one = array(
		"Category" => "Category",
	);

	private static $has_many = array(
		"Services" => "Service"
	);
	//...
}

//...

class Category extends DataObject {
	//...
	private static $has_many = array(
		"Businesses" => "Business",
		"Services"   => "Service"
	);
	//...
}

//...

class Service extends DataObject {
	//...
	private static $has_one = array(
		'Category' => 'Category'
	);

	private static $belongs_many_many = array(
		"Businesses" => "Business",
	);
	//...
}