[WIP] Add normalizePayload(modelName, payload) to DS.Store

[pangratz]

A payload can be normalized by store.normalizePayload() into the JSON
API format Ember Data expects for store.push. The method optionally
takes a primary model name as first argument. If a primary model is
specified, the normalized payloads' primary data contains data for this
model from the payload and all other data is included.

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This addresses #3605.

Opened this PR to get some feedback if I understood #3605 correctly and if this is heading the right direction...

[bmac]

I like this change. @pangratz do you mind wrapping this code in feature flags?

[workmanw]

Just a small piece of feedback on the naming. As someone who maintains a custom adapter and serializer, this is kind of makes my head spin. There was a normalizePayload on the JSONSerializer previously, which was deprecated and renamed to normalizeResponse in Ember-Data 1.13. The old normalizePayload would take a server response and normalize it via the serialize. And now I'm still not totally sure what the difference between the current normalizeResponse and the new normalizePayload will be.

[bmac]

Thanks @workmanw. That is excellent feedback. I've flagged this feature for discussion at the Ember Data weekly meeting to hopefully clarify its role, naming and necessity in the system.

[bmac]

We chatted about this method in the Ember Data weekly meeting and were a little skeptical about the benefits of adding normalizePayload without a clear overall understanding of the problem space. I think the next steps here are to look enumerate the problems that store#pushPayload, store#normalize and store#normalizePayload solve and try to figure out if there are any gaps or places where APIs can be simplified.

Making pushPayload return a value as @bcardarella suggested in #3576 may also be an acceptable solution to this problem.

[bmac]

@pangratz I'm going to close this because I think it would be better see pushPayload return a value to solve this issue.

[pangratz]

👍