REST API one-to-many resource relationships

I hope I understood your question correctly.

As far as I am concerned, CI does not inherently support one-to-many relation between relational database rows in its DB / entity logic. I myself solved this by creating a sort of higher-order database entity class that takes care of that.

As a side note, I do not expect CI to support it directly, as, in my opinion, it is beyond the scope of this lightweight framework.