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.

Start by reviewing the resource relationship declarations (relationship, has_many, and has_one) and the InvalidInclude path described in the issue. Determine how a public method such as Video#related_videos could be represented and exposed through the two example requests; done criteria require agreement on the API and validation behavior.