I need help understanding data selectors
I help run an online course registration/payment system for a small college. We're currently using Ubercart, but I think a redesign of our store coupled with DrupalCommerce and its integration with Rules will solve some of the issues we're having.
In particular, I want to keep the user from being able to accidentally do some things like overbook themselves (sign up for two classes that meet at the same time).
I think I might be able to do this with Rules. When the customer tries to add an item to their cart I want to check to see if they already have an item in their cart that meets at the same time slot.
But in order to do that I really need to understand data selectors better. I have searched high and low, and maybe I just missed something, but I cannot find a good explanation of data selectors. Any help would be hugely appreciated.