Ember.js: Computed property from child models

in my Ember.js application I have models: item, module, rank with following structure: item hasMany modules, each module can have a rank and rank has property score.

Item is my model for controller. I want to show sums of all scores from all modules. So I want to create computed property, however I have no idea how to write the dependency in .property("name_of_property"). Could you help me?