Use links not keys to represent relationships in APIs

But links can appear in API resources too, and using them instead of foreign keys significantly reduces the amount of information that has to be separately documented by the API provider and learned by the user. The relationship name in a link can be thought of as being analogous to the column name of a relational database foreign key column, and the URI in the link is analogous to the foreign key value. While links aren’t widely used in APIs, some very prominent web APIs use links based on HTTP URLs to represent relationships, for example the Google Drive API and the GitHub API.

Source: cloudblog.withgoogle.com