Right, only querying a few fields seems not to be a builtin feature. Looks like you have to create the partial selectable type yourself and there is no support to typecheck that the correct columns in the select are included.
Your second case, if I recall this correctly (ActiveRecord made my SQL skills fade away), this plain JOIN would just return a row with the same book but a different author. `to_jsonb(authors.*)` is just operating on a single row. But what you want is possible (aggregating rows into a JSON object) by using `jsonb_agg`. Whether the lib supports inferring the correct typings for that is another question though.
Your second case, if I recall this correctly (ActiveRecord made my SQL skills fade away), this plain JOIN would just return a row with the same book but a different author. `to_jsonb(authors.*)` is just operating on a single row. But what you want is possible (aggregating rows into a JSON object) by using `jsonb_agg`. Whether the lib supports inferring the correct typings for that is another question though.