This looks pretty great and I'd definitely consider adding it to new projects, but it's not a magic bullet. You still need to think about what queries this will

This looks pretty great and I'd definitely consider adding it to new projects, but it's not a magic bullet. You still need to think about what queries this will trigger and it's a lot less obvious than includes(). If you have a large application with an ORM like Active Record, you'll often end up in situations where queries inside methods trigger the N+1 problem. Sometimes this loading is highly conditional. You end up trading the N+1 problem for a "loading an entire unnecessary table" problem. I suspect this is the reason this feature hasn't been added to Active Record. It would be used a lot when it's only narrowly useful and has worse side effects. (It's also yet one more layer of complexity on top of an already complex ORM.)