Magento intercept coupon code logic?

Question

Trying super hard to understand Magento's code.

I need a way to intercept coupon codes to prevent them working if an item in the cart is already discounted.

How can I achieve this?

Answer 1

This can be achieved by using an observer, subscribing to the controller_action_predispatch_checkout_cart_couponPost event.

Or by overriding the Cart controller. Specifically, the couponPostAction method