dlayer-knex/src/dataloader-from-knex-results.js

"use strict";

const DataLoader = require("dataloader");
const syncpipe = require("syncpipe");

module.exports = function dataloaderFromKnexResults({ fetch, selectKey }) {
	/* A `whereIn` query would return any results that were found, but *not* produce empty
	 * results for those that weren't. But DataLoader expects for exactly one item to be
	 * provided for each supplied ID, even if it doesn't exist.
	 * 
	 * This abstraction deals with that by collecting the results from Knex, and then
	 * creating a DataLoader-compatible sequence that pulls from these results, leaving
	 * gaps for non-existent items.
	 */

	let selectKeyCallback = selectKey ?? ((item) => item.id);

	return new DataLoader(async (keys) => {
		let matches = await fetch(keys);
	
		let byKey = syncpipe(matches, [
			_ => _.map((record) => [ selectKeyCallback(record), record ]),
			_ => Object.fromEntries(_)
		]);
		
		return keys.map((key) => byKey[key] ?? null);
	});	
};