Ember.js: Changing `firstObject` does not cause a rerender.

[Serabe]

Given the following template:

{{#each collection as |item|}}
  {{item}}
{{/each}}

Setting collection.firstObject to a different value does not rerender.

Twiddle

[chancancode]

Is that a supported way to mutate an ember array? o_0 cc @rwjblue

[rwjblue]

I would not expect someArray.set('firstObject', 'foo'); to set the first item today, though I do suppose that if you think about it when you call someArray.get('firstObject') it works so it is somewhat surprising that you can't set it...

[Serabe]

I would expect that the call either sets it or raises a readOnly assertion (Cannot set read-only property firstObject on object: x).

[rwjblue]

@Serabe - Ya, I agree that silently failing isn't good. I would lean towards making it a readOnly error I think...

Open to suggestions from @krisselden / @mmun though...

[mmun]

Setting firstObject should raise an error, yes. It might be possible that this introduces errors in people's apps where there weren't any previously, e.g. in {{foo-bar baz=array.firstObject}} because of the two-way binding, but I'd guess it would be pretty rare.

[rwjblue]

Ya, it seems rare and also a likely source for other bugs...

[pixelhandler]

Perhaps some clarity can be added to the API docs for firstObject that it should not be set. The doc does mention that its a helper method to get something. But since its listed as a property one would think that it's ok to get and set. Which is not the case.