Observer causing Maximum call stack size exceeded with nested fragments

I am using
ember data: 2.1.0
ember: 2.1.0
ember model fragments: 2.1.2

I have a model with a fragment that has a fragment. When using a computed alias or an observer, I get a Maximum call stack size exceeded after saving.

here is the full stack track:
`RangeError: Maximum call stack size exceeded
at GETTER_FUNCTION (http://localhost:3000/assets/item-bank/assets/vendor.self-722485c363562af5f2b685ca5777edffe70105b854483e9df6b9b4133333cc09.js?body=1:32662:36)
at get (http://localhost:3000/assets/item-bank/assets/vendor.self-722485c363562af5f2b685ca5777edffe70105b854483e9df6b9b4133333cc09.js?body=1:33144:27)
at _getPath (http://localhost:3000/assets/item-bank/assets/vendor.self-722485c363562af5f2b685ca5777edffe70105b854483e9df6b9b4133333cc09.js?body=1:33234:14)
at Object.get (http://localhost:3000/assets/item-bank/assets/vendor.self-722485c363562af5f2b685ca5777edffe70105b854483e9df6b9b4133333cc09.js?body=1:33149:14)
at Descriptor.AliasedProperty_get [as get] (http://localhost:3000/assets/item-bank/assets/vendor.self-722485c363562af5f2b685ca5777edffe70105b854483e9df6b9b4133333cc09.js?body=1:26390:36)
at Object.get (http://localhost:3000/assets/item-bank/assets/vendor.self-722485c363562af5f2b685ca5777edffe70105b854483e9df6b9b4133333cc09.js?body=1:33153:19)
at lazyGet (http://localhost:3000/assets/item-bank/assets/vendor.self-722485c363562af5f2b685ca5777edffe70105b854483e9df6b9b4133333cc09.js?body=1:27241:38)
at Object.ChainNode.value (http://localhost:3000/assets/item-bank/assets/vendor.self-722485c363562af5f2b685ca5777edffe70105b854483e9df6b9b4133333cc09.js?body=1:27255:23)
at Object.ChainNode.notify (http://localhost:3000/assets/item-bank/assets/vendor.self-722485c363562af5f2b685ca5777edffe70105b854483e9df6b9b4133333cc09.js?body=1:27391:32)
at Object.ChainWatchers.notify (http://localhost:3000/assets/item-bank/assets/vendor.self-722485c363562af5f2b685ca5777edffe70105b854483e9df6b9b4133333cc09.js?body=1:27128:18)`

No repository file or test is named in the report. First reproduce with Ember Data 2.1.0, Ember 2.1.0, and ember-data-model-fragments 2.1.2 using nested fragments plus an observer or computed alias, then trace the reported getter and chain-watcher stack. Done means saving the nested-fragment model no longer causes a maximum call stack error.

索引モデルが issue の本文から書いたものです。