Writing directives to handle input of collections (lists or objects) is currently not very well supported because ngModelController uses identity tests to determine if the model or view value changes.

Adding a collection watch mode to ngModelController would allow for more robust input directives such as checkbox lists to work without having to do things like creating new lists on every update or monkey patching the ngModelController instance.

This issue has come up a few times, but the only place I could find it yesterday was in #2553, which is closed.