CanCanCan: Defining Abilities

I'm using the CanCanCan gem. According to the documentation you can define abilities. That is working for me. What I want to do is to limit the access to records, that contain a value. Something like:

"can :crud, Order, :brand == nil"

is not working

A User should only be allowed to see the record, if the brand-column of the Order table is empty. As my example doesn't work apparently.. How would you do it?