Angular.js Checkboxes

I'm trying to do something very simple with checkboxes and Angular 1.x. I want to pre-select some checkboxes on an edit page and then after some selections are made to save the data to a backend.

I have created this demo:

http://jsfiddle.net/roL38ctk/3/

I'm able to pre-select the checkboxes but I struggle to see how the selected items or of any use and could ever be saved to a database, for example after checking and un-checking the options my ng-model might look like this:

{"300":false,"400":true}

When saving I need to know the ids which have been selected, I have no need or desire to know the state of the checkboxes. Something like:

{300,305,310}

To me it seems that Angular doesn't handle checkboxes very well without having to hack objects together to extract and massage the data ready for saving.

As a guide I have used http://embed.plnkr.co/g0NMG4rmF4uwzoG2uZhf/preview which I believe is reputable source and claims to be the Angular way? I'm disbelieving that something some elementary can be so taxing when a PHP framework manages this effortlessly. Where am I going wrong?