The association would still exist implicitly in the code that uses the database. The columns could stay the same, and the code could still use the item's hypoth

The association would still exist implicitly in the code that uses the database. The columns could stay the same, and the code could still use the item's hypothetical cart_id column to get the items in a cart. You lose the explicitness and safety of referential integrity in the database, but it's a trade off that can enable other things, like sharding, as mentioned by the GitHub employee.