DS.Store#findHasMany()

If a relationship was originally populated by the adapter as a link (as opposed to a list of IDs), this method is called when the relationship is fetched.

The link (which is usually a URL) is passed through unchanged, so the adapter can make whatever request it wants.

The usual use-case is for the server to register a URL as a link, and then use that URL in the future to make a request for the relationship.