Cocoa binding with @sum value of child

I'm trying to have a table column to display the sum of child values using the IB. I'm not sure if this is 开发者_运维百科doable though.

So I have my Core Data bind to a table and a column of "total sales amount" that should be a total of the sales of the parent object.

I'm trying to do it like this:

"departments.@distinctUnionOfSets.sales.@sum.amount" in the "Model Key Path" of the Table Column Binding in IB.

Is there any example or tutorial on how to do this.