Angular Material checkbox group issues

Allow md-checkbox to iterate over array of elements as follows:

<md-checkbox ng-repeat="cat in categorias" value="{{cat.id}}" ng-checked="{{cat.id.indexOf(item.cats) > -1}}" ng-model="item.cats">{{cat.name}}</md-checkbox>

and populate model with a list of selected values such as http://ift.tt/1lmEPlk

Feedback appreciated...

Plunker http://ift.tt/1CQ6oR2