Trying to automatically add a product in Commerce

Question

I'm trying to add a product to a cart (and emptying the cart first) if a user just visits a template. My goal is to make a one-page checkout: so I want the address info on the product page itself. This is what I have so far. I thought the first line would make a cart if there is no cart yet, but it seems nothing happens. (cart.id = null)

{% set cart = craft.commerce.carts.cart %}
Cart ID is {{ cart.id }}
{% do cart.setLineItems([]) %}
{% set lineItem = craft.commerce.lineitems.resolveLineItem(cart.id, product.id, [], 1, '') %}
{% do cart.addLineItem(lineItem) %}

Answer 1

I tried this myself and it threw an error when there was no cart attached to the session.

I then found these two lines of code in the Craft Commerce CartController.php:

        // Get the cart from the current users session, or return a new cart attached to the session
        $cart = Plugin::getInstance()->getCarts()->getCart(true);

getCart() can be found in the Carts Service

Which lead me to try:

{% set cart = craft.commerce.carts.cart(true) %}

So if you pass in true, you force it to save the cart and thus you seem to be able to create a new (empty) cart.