Related resources not backed by an association

Hello, I'm curious to see if anyone is interested in allowing a resource to have a relationship that isn't necessarily an association to the underlying model. For example:

A Video has_many :tags. I've created an instance method (Video#related_videos) that pulls related videos based on a video's set of tags.

video.related_videos
=> [related_video_1, related_video_2]

What I'm interested in is having the corresponding VideoResource aware that this isn't an association it should be looking for, but rather public method. This would allow the following requests:

videos/:id?include=related_videos
videos/:id/relationships/related_videos

Currently, an InvalidInclude exception will be raised since "related_videos is not a valid relationship of videos." What if we were to pass a flag to relationship / has_many / has_one? Maybe there's a better way, but I was thinking something like:

VideoResource < JSONAPI::Resource
  has_many :related_videos, instance_method: true
end

Would something like this be worth exploring? I'd be happy to work on a PR to support something like this if there's any interest.

まず、issue で説明されているリソース関係の宣言（relationship、has_many、has_one）と InvalidInclude パスを確認します。Video#related_videos のような public メソッドを 2 つのリクエスト例を通じてどのように表現し公開できるかを判断します。完了条件では、API とバリデーションの動作について合意する必要があります。