Ember Correct way to build computed property for an array or arrays

Demonstration at http://ift.tt/21gegkU

I'm trying to build a computed property that depends on items within an array or arrays. Logically this could be done with array1.@each.array2.[] but the documentation is clear that is not valid syntax and results in a warning even though it works.

The warning and guide both tell me to create an interim computed property to bridge this gap, but I can't logically see how to create the property and not run into the exact same problem.