SQL.md

Using DataLoader with SQLite

While not a key-value store, SQL offers a natural batch mechanism with SELECT * WHERE IN statements. While DataLoader is best suited for key-value stores, it is still suited for SQL when queries remain simple. This example requests the entire row at a given id, however your usage may differ.

const DataLoader = require('dataloader');
const sqlite3 = require('sqlite3');

const db = new sqlite3.Database('./to/your/db.sql');

// Dispatch a WHERE-IN query, ensuring response has rows in correct order.
const userLoader = new DataLoader(
  ids =>
    new Promise((resolve, reject) => {
      db.all(
        'SELECT * FROM users WHERE id IN $ids',
        { $ids: ids },
        (error, rows) => {
          if (error) {
            reject(error);
          } else {
            resolve(
              ids.map(
                id =>
                  rows.find(row => row.id === id) ||
                  new Error(`Row not found: ${id}`),
              ),
            );
          }
        },
      );
    }),
);

// Usage

const promise1 = userLoader.load('1234');
const promise2 = userLoader.load('5678');
const [user1, user2] = await Promise.all([promise1, promise2]);
console.log(user1, user2);