Referencing Get entire collection data from relation widget - #9 by erez for context.
You could iterate during build time over the collection that contains the relation widget (the one with the - {label: "Author", name: "author", widget: "relation", collection: "authors", searchFields: ["title"], valueField: "title", displayFields: ["title"], required: true} field), get the author value, then use it to query the author data.