I need to design data model for combo discount

I have the following scenario for -

item1 + item2 => 10% discount

item1 + item3 => 12% discount

item2 + item4 => 11% discount and so on…

The number of items in combination could be variable. eg:

item1 + item2 + item3 + item4 => 15%discount

Can someone provide guidance about how to approach this problem using mongodb?