createLoaders.js

import DataLoader from 'dataloader';

function indexResults(results, indexField, cacheKeyFn = key => key) {
  const indexedResults = new Map();
  results.forEach((res) => {
    indexedResults.set(cacheKeyFn(res[indexField]), res);
  });
  return indexedResults;
}

function normalizeResults(keys, indexField, cacheKeyFn = key => key) {
  return (results) => {
    const indexedResults = indexResults(results, indexField, cacheKeyFn);
    return keys.map(
      val =>
        indexedResults.get(cacheKeyFn(val)) ||
        new Error(`Key not found : ${val}`),
    );
  };
}

const cacheKeyFn = key => key.toString();

async function knexLoader(knex, name, ids) {
  const results = await knex(name).whereIn('id', ids);
  return normalizeResults(ids, 'id', cacheKeyFn)(results);
}

class Loaders {
  constructor(knex) {
    this.loaders = {};
    this.knex = knex;
  }

  table(name) {
    if (this.loaders[name]) {
      return this.loaders[name];
    }

    this.loaders[name] = new DataLoader(ids =>
      knexLoader(this.knex, name, ids),
    );

    return this.loaders[name];
  }
}

export default function createLoaders(knex) {
  return new Loaders(knex);
}