Rails: two relations same model design

Conceptual question:

I have a regular e-commerce application with models Order, and User. Order has a column status that indicates the status of the order. In particular, and what's relevant to this question is that status = 0 indicates a cart.

Now while each User has many Orders, each User typically has one Cart.

For this in my User model, I have defined:

has_many :orders
has_one :cart, -> { where "status = 0" }, class_name: 'Order'

My trouble with this is that my various controllers, I can't use:

cart = my_user.cart || my_user.cart.build

The best that I can hope for is:

cart = my_user.cart || my_user.orders.build(status: 0)

Also the current implementation makes ensuring that only a single cart exists (one order with status 0) difficult. Now I'm relatively new to Rails, and still familiarizing myself with recommended design practices. What's the best way to achieve this functionality in Rails?