buy "y" only if you buy "x"

Question

I have in the shop 2 categories x and y. Customer has to have the ability to:

1. buy products from category "x" normally
2. purchase from category "y" should be locked. He cant buy it separately
3. From cat. "y" he can buy only if he buy from cat. "x"

Products from categories "x" and "y" must be visable, and have qty>0

Answer 1

Just implement an observer which checks on checkout_cart_product_add_after whether the dependency is fulfilled, it is not, remove the Product Y from cart.