Introduces per-request adapter switching based on the request mime.

[rescribet]

Purpose

The current behaviour is to always use the same adapter, this is not very desirable in more formal hypermedia conforming systems where content-negotiation determines the format which should be used. This PR introduces the option to let the system choose the adapter based on the negotiation results.

Changes

Content-type determination is left to ActionController::MimeResponds, so that users won't have to specify the adapter in every render call.

Caveats

• This implementation uses a non-existent virtual adapter :mime to let the subsystem know to look up the adapter dynamically, which may be considered inconsistent.
• By using MimeResponds, an implicit dependency between mime types and serializers is created, which might not be suitable for projects needing to use different adapters under the same mime type.
• I'm not aware how these changes will affect view caching.

Related GitHub issues

@remear's comment on PR #1082 was the cause to reimplement this feature.
#1039

[mention-bot]

@Fletcher91, thanks for your PR! By analyzing the history of the files in this pull request, we identified @bf4, @domitian and @bacarini to be potential reviewers.

[bf4]

IMO, only the documentation here and some tests are required since request.format is rails-speak for the negotiation mime type, which, for JSON:API is format.jsonapi, which uses render jsonapi: model

Please feel free to disagree and point out anything I'm not considering

[bf4]

👍

should be jsonapi not json_api, and render jsonapi: model rather than render json: model.

But really, this is just using the content type negotiation that respond_to already does. This would work without any changes to the PR, assuming mime -> adapter is 1:1

[rescribet]

Would you prefer just a correction:

To automatically switch between adapters based on the request, set the adapter to :mime:

  # In configuration
  ActiveModelSerializers.config.adapter = :mime
  
  # In controller action
  respond_to do |format|
    format.json { render json: Model.first }     # Uses the JSON adapter
    format.jsonapi { render jsonapi: Model.first } # Users the JSON:API adapter
    format.bug { render json: Model.first } # Will raise `UnknownAdapterError`
  end

or to just leave it implicit:

To automatically switch between adapters based on the request, set the adapter to :mime;

ActiveModelSerializers.config.adapter = :mime

[bf4]

Really, if you want to render different mime types, each mime type should be registered to its own formatter. That way, you can do the below, which is the Rails-way IMHO and requires no changes to the codebase.

 # In configuration
ActiveModelSerializers.config.adapter = :json
  
# In controller action
respond_to do |format|
    format.json { render json: model, serializer: ModelSerializer }     # Uses the JSON adapter
    format.jsonapi { render jsonapi: model, serializer: ModelSerializer } # Users the JSON:API adapter
    format.bug { render json: model, serializer: ModelSerializer } # Will raise something about bug not being a known mime type
  end

The respond_to block does the content type negotiation for you and usually we use a renderer format with the same name as the negotiated / acceptable mime types.

I think I'm making different assumptions than you, so please continue to discuss.