Modify Plugs.DataToAttributes to follow JSON API sideposting draft

Issue で参照されている DataToAttributes コードから始め、JaSerializer.Params.to_attributes が belongs-to リレーションをどのように処理しているかを確認してください。動作を変更する前に、JSON API sideposting draft と issue #1301 を確認してください。完了条件は、array リレーションが temp-id 識別子を使用して一致する included レコードを選択し、map リレーションは引き続き JaSerializer.Params.to_attributes で処理されることです。

Problem

From https://github.com/json-api/json-api/pull/1197/files?short_path=571f6ce#diff-571f6cee9a492e0e177801c21a4b2b2a

Our current approach with included records is to simply assume they are related to the main record.

Based on the draft

• included record payloads should have temp-id properties if they are to be created.
• the main record payload should have a relationship where the identifier is %{data: [%{"temp-id" => "foo", "type" => "bar"}
• the plug should get those identifiers, then replace the identifier payloads with the associated full payloads matched in included

Subtasks

• instead of just taking any included records and assuming they're relationships in the resulting params map, DataToAttributes should parse the relationships map first
  • the relationship where data is a map is a belongs to and should be left alone. it's already handled by JaSerializer.Params.to_attributes
  • the relationship where data is an array should be checked for identifiers with temp-id keys
    • each temp-id should be found in the included section and added under the pluralized relationship key in the resulting map

An advantage of this is that we now are able to conclude it's definitely a has_many, so we do not need to explicitly specify this anymore.

References

The code this needs to modify is part of #1301 so this is blocked until that is merged.