observing property changes to an ObjectProxy in Ember

Following this tutorial on how to use checkboxes with ember: http://ift.tt/1H1gPOT.

Everything works great. Except now I'd like to listen for when the property 'checked' changes in the object. Here is my code:

someArray: Ember.computed 'proxiedCheckedItems.[]', ->
    // should fire whenever an item is checked. But it does not.
    console.debug("CD: detected change here")
    // return some array

proxiedModel: Ember.computed.map 'someArray', (model) ->
    Ember.ObjectProxy.create({
        content: model,
        checked: false
    })

proxiedCheckedItems: Ember.computed.filterBy('proxiedModel', 'checked', true)

so, observing proxiedCheckedItems.[] is wrong. I've tried proxiedCheckedItems.@each and proxiedCheckedItems.checked. Nothing works. So how do you observe an array of ObjectProxy in Ember?