Protractor: creating an array from nested elements

I'm trying to get elements in my DOM and return them as an array. They are nested. I want protractor to return something like this:

{
 category_title: "cat1",
 items: {item_title: "item1"}, {item_title: "item2"}
},
{
 category_title: "cat2",
 items: {item_title: "item1"}, {item_title: "item2"}
}

DOM:

<div ng-repeat="category in categories">
    <div id="category-title">{{ category.title }} </div>
    <div ng-repeat="item in category.items">
        <div id="item-title">{{ item.title }} </div>
    </div>
</div>

This is what I'm trying in Protractor:

return element.all(by.repeater("category in categories")).map(function(elm) {
    var category_title = elm.element(by.id('category-title')).getText(),
    var t = {
              category_title: category_title,
              dimensions: {}
            };
    // After this part, I'm lost...
    return elm.all(by.repeater("item in category.items")).map(function(elm) {
        return {
                 category_title:  category_title,
                  dimension:  elm.element(by.id('item-title')).getText(),
               }
        });
});