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DataLoader classes to make it easier to write complex graphql resolvers.

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A Factory pattern designed to be used to implement GraphQL resolvers efficiently.

The basic concept of this library is that you will create a new factory instance per request and stash it in the graphql request context. Then you can ask the factory any time you need a dataloader instance, and it will either generate one or return one it already created.

In addition, it takes on the burden of putting results into the correct order for dataloader, by having you specify an extractId or extractKey function to teach it where the id is in your data object.

Basic Usage (Load by Primary Key)

You can spread your *Loader configurations out into any file structure you like, just create and export an instance of each type of loader, so you can import it in your resolvers.

import { PrimaryKeyLoader } from 'dataloader-factory'
export const authorLoader = new PrimaryKeyLoader({
  fetch: async ids => {
    return db.query(`SELECT * FROM authors WHERE id IN (${ => '?').join(',')})`, ids)
  extractId: book =>

Then the factory should be added to context on each request, e.g.:

import { DataLoaderFactory } from 'dataloader-factory'
new ApolloServer({
  context: req => {
    return { dataLoaderFactory: new DataLoaderFactory() }

Then it may be used in resolvers:

import { authorLoader } from './authorLoader.js' // or wherever you put it
export const bookAuthorResolver = (book, args, context) => {
  return context.dataLoaderFactory.get(authorLoader).load(book.authorId)

When you pass an instance of one of the *Loader classes to dataLoaderFactory.get(), dataloader-factory will use the configuration information it contains to create and cache instances of DataLoader for you, generating a brand new set of DataLoaders for every new request.


The PrimaryKeyLoader constructor accepts the following input:

const myLoader = new PrimaryKeyLoader<IdType, ObjectType>({
  // the core batch function for DataLoader, except DataLoaderFactory handles
  // putting it back together in order, so all you need to do is fetch
  // see below for discussion of context parameter
  fetch: (ids: IdType[], context) => ObjectType[],

  // a function for extracting the id from each item returned by fetch
  // if not specified, it guesses with this default function
  extractId: item => || item._id,

  // specify one or more primary key loaders and they will automatically
  // be primed with any results gathered
  // NOTE: if the above fetch function returns result objects that differ from those of
  // the specified loader(s), it's going to cause you problems
  idLoader: PrimaryKeyLoader|PrimaryKeyLoader[],

  // the options object to pass to dataloader upon creation
  // see dataloader documentation for details
  options: DataLoader.Options

Note about extractId

Let's take a moment to discuss the extractId configuration option as shown in the above example. I find that it is a common sticking point for people first learning about building graphql APIs with dataloader-factory.

If you've read the documentation for dataloader (and I hope you have!), you'll recall that the batch function you give to new DataLoader is required to return results in the exact same order as the keys that were provided, and the return array should have undefined in it for any keys that didn't turn out to exist.

So if you get the array [2, 1, 3], and 3 doesn't exist in your database, you must return [{ myKey: 2, ...moredata }, { myKey: 1, ...moredata }, undefined]

The typical performant pattern for that is:

const myKeyLoader = new DataLoader(async (keys) => {
  const rows = await lookupKeysInDatabase(keys)
  const rowsByKey = results.reduce((rowsByKey, row) => ({ ...rowsByKey, [row.myKey]: row }), {})
  return => rowsByKey[k])

I find that pattern to be quite repetitive and ugly, even if you replace the reduce with lodash.groupBy or the like. Furthermore, the one-to-many and many-to-many patterns are a little more complicated to unwind. So dataloader-factory handles that part for you. Your fetch function only has to return a set of rows, and they'll get properly sorted and formatted for dataloader automatically.

The only thing I need from you to make that happen is a function that tells me which property represents the key that we were batching on. That's why extractId (or extractKey(s) in the other loader types) is part of the configuration. In the example above, we were batching on myKey so the extractId function would be row => row.myKey.

You can also specify a string extractId: 'myKey' instead of a function and that'll work. Or if your key is the highly typical 'id' or '_id', you don't even need to specify extractId because I'll look for those anyway (in that order of preference).

Our above pattern becomes much simpler:

const myKeyLoader = new PrimaryKeyLoader({
  fetch: async (keys) => await lookupKeysInDatabase(keys),
  extractId: row => row.myKey

Pass-through Context

In the Options section above, you may have noticed that the fetch function receives a context parameter. This is an extra option for you to be able to pass information from the request context through to your fetch function, in case you have filters that are context-sensitive, like a dataloader for fetching friendships of the current user or one that needs to fetch only records the current user is authorized to read.

To set the pass-through context, pass it in when you construct a new dataLoaderFactory instance, and then it will be passed to your fetch functions.

new ApolloServer({
  context: req => {
    const userinfo = parseBearerToken(req.headers.authorization)
    return { dataLoaderFactory: new DataLoaderFactory(userinfo) }

Note about idLoader

The idLoader configuration is there to help you keep your dataloaders properly primed. For instance, say you have a loader for fetching books by their authorId. Well, those books that got returned have all their data, so there's no reason why you should go back to the database if a later part of the query asks for one of the books by id. To make sure we can re-use books fetched by authorId, we need to prime the id loader with the results of the authorId loader. We do that with the idLoader configuration option on the authorId loader:

const booksByIdLoader = new PrimaryKeyLoader({ ... configuration ... })
const booksByAuthorIdLoader = new OneToManyLoader({
  // ... regular configuration for the loader ...
  idLoader: booksByIdLoader

Now dataloader-factory will automatically keep booksByIdLoader primed with anything fetched by booksByAuthorIdLoader.

It's also possible that you have two good unique keys for a data type. For example, a book might have an internal auto_increment id in your system, plus its ISBN identifier used by the rest of the world. You probably want to have the book cached no matter which identifier gets queried.

It's a little more complicated to pull this off because you can't set the idLoader on whichever loader you make first, because the second loader won't exist yet. For this case, PrimaryKeyLoader offers a method .addIdLoader(loader) so you can add it after creation:

const booksByIdLoader = new PrimaryKeyLoader({ /* configuration */ })
const booksByISBNLoader = new PrimaryKeyLoader({
  // ... configuration ...
  idLoader: booksByIdLoader

One-to-Many DataLoaders

The PrimaryKeyLoader is only appropriate for primary key lookups (or another key that identifies exactly one record). To fetch relations that return an array, a more complex pattern is required.

The first of these patterns is the one-to-many pattern. Use it when you have a one to many relationship you're trying to map, and your fetch function returns objects that will each map to a single key value. For instance, a page always exists inside a single book, so the pagesByBookId implementation might look like this:

import { OneToManyLoader } from 'dataloader-factory'
const pagesByBookIdLoader = new OneToManyLoader({
  fetch: async bookids => {
    return db.query(
      `SELECT * FROM pages WHERE bookId IN (${ => '?').join(',')})`,
  extractKey: page => page.bookId

The resolver might then look like this.

export const bookPagesResolver = (book, args, context) => {
  return context.dataLoaderFactory.get(pagesByBookIdLoader, args).load(

Note that this is also useful for many-to-many relationships that have a named intermediary. For instance, the relationship between a book and a library might be represented as an Acquisition that links a book and a library and additionally lists a date the book was purchased. In this case the dataloader for book -> acquisition is one-to-many, the dataloader for library -> acquisition is one-to-many, and for book -> library the developer has the option of chaining book -> acquisition -> library or creating a new many-to-many dataloader that uses a database join to save a round trip (see the "Many-to-Many-Joined" section below).


The OneToManyLoader constructor accepts the following inputs. All of the *-to-many patterns accept the same options, except as specifically noted in their section of the documentation.

const myOneToManyLoader = new OneToManyLoader<KeyType, ObjectType, FilterType>({
  // accept arbitrary foreign keys and arbitrary arguments and return results
  // the keys MUST appear in the result objects so that your
  // extractKey function can retrieve them
  // see PrimaryKeyLoader options for discussion of context parameter
  fetch: async (keys: KeyType[], filters: FilterType, context) => ObjectType[] // required

  // function that can pull the foreign key out of the result object
  // must match the type of the keys you're using in your fetch function
  extractKey: (item: ObjectType) => item.authorId // required

  // advanced usage only, covered later in this readme
  matchKey: (key: KeyType, item: ObjectType) => boolean

  // see below section titled "Note about filter and batch overlap"
  keysFromFilter: (filters: FilterType) => KeyType[]

  // generated dataloaders will not keep a cache, batch only
  skipCache: false

  // maxBatchSize to be passed to each DataLoader
  maxBatchSize: 1000

  // cacheKeyFn to be passed to each DataLoader, default is fast-json-stable-stringify
  // which should be good for almost any case
  cacheKeyFn: (key: KeyType) => stringify(key)

  // specify one or more primary key loaders and they will automatically
  // be primed with any results gathered
  // NOTE: if the above fetch function returns result objects that differ from those of
  // the specified loader(s), it's going to cause you problems
  idLoader: PrimaryKeyLoader|PrimaryKeyLoader[]

Note that KeyType can be anything serializable, so you can use arrays or objects for any compound keys you may have.

Many-to-Many DataLoaders

For DataLoaderFactory, the Many-to-Many pattern is split into two use-cases: one targeted at document-oriented databases like MongoDB (this section), another for relational databases like MySQL or Oracle (see the next section, "Many-to-Many-Joined").

In document-oriented databases a typical pattern for a simple many-to-many relationship is to store an array of keys inside one of the documents. For instance, a book might be represented like this:

  id: 1,
  title: 'Great American Novel',
  genreIds: [1,3,8]

The Book.genres resolver is trivial, you can use the primary key loader with your genres array. However, Genre.books requires a special treatment from DataLoaderFactory that asks you for extractKeys instead of extractKey (all other options are identical):

import { ManyToManyLoader } from 'dataloader-factory'
const booksByGenreIdLoader = new ManyToManyLoader({
  fetch: async genreIds => {
    // this example is for mongodb client
    return db.collection('books').find({ genreIds: { $in: genreIds } }).toArray()
  extractKeys: book => book.genreIds

and the resolver

export const genreBooksResolver = (genre, args, context) => {
  return context.dataLoaderFactory.get(booksByGenreIdLoader).load(

Note that it is also possible to use a named intermediary in document-oriented databases. Depending on the database, you may still find the Many-to-Many-Joined pattern useful in those cases.

Many-to-Many-Joined DataLoaders

It is technically possible to handle SQL many to many relationships with the OneToManyLoader, like this:

import { OneToManyLoader } from 'dataloader-factory'
const booksByGenreIdLoader = new OneToManyLoader({
  fetch: async genreIds => {
    const books = await db.get(`
      SELECT b.*, bg.genre_id as genreId
      FROM books b
      INNER JOIN books_genres bg ON = bg.book_id
      WHERE bg.genre_id IN (${ => '?').join(',')})`)
    return books
  extractKey: book => book.genreId

This will work but it means the book object being passed to the rest of your code has a genreId property that doesn't really belong there. This is especially annoying when using Typescript as you need to create a new interface like BookWithGenreId to represent this not-quite-a-book object.

Luckily DataLoaderFactory provides a cleaner pattern with ManyJoinedLoader:

import { ManyJoinedLoader } from 'dataloader-factory'
const booksByGenreIdLoader = new ManyJoinedLoader({
  fetch: async genreIds => {
    const books = await db.get(`
      SELECT b.*, bg.genre_id as genreId
      FROM books b
      INNER JOIN books_genres bg ON = bg.book_id
      WHERE bg.genre_id IN (${ => '?').join(',')})`)
    return => ({ key: row.genreId, value: new Book(row) }))

You no longer provide an extractKey function because you return it with each row. DataLoaderFactory will use the key you provide to put the data back together and then discard it, returning only the pristine Book from the value field back to the .load() call in your resolver.

Best-Match DataLoaders

In some rare cases, you may have a key that provides a fuzzy match to an object. For instance, to create a self-healing link, you might have a key like { id: 9, path: '/about' }. You're looking for an object that has id 9, or if nothing has id 9, then a an object with path /about.

This is really complicated to dataload, so this library provides a BestMatchLoader especially for this case.

Here's an example where I have a book's id, title, and author, but I'm not sure whether the id has changed in my database. So I want to fall back to title and author when id doesn't match.

import { BestMatchLoader } from 'dataloader-factory'
const bookByLinkLoader = new BestMatchLoader({
  fetch: async (keys: { id: string, title: string, author: string }[]) => await db.get(`
      SELECT * FROM books
      WHERE id IN (${ => '?').join(',')})
      OR (title, author) IN ((${ => '?,?').join('),(')}))
    `, [ =>, ...keys.flatMap(key => [key.title,])]),
  scoreMatch: (key, book) => ===
    ? 2
    : ( === && key.title === book.title
        ? 1
        : 0)

Then I can use the loader like normal, expecting AT MOST ONE result per key (it can still end up with no matches).

export const libraryItemLinkResolver = (libraryItem, args, context) => {
  const book = await context.dataLoaderFactory.get(bookByLinkLoader).load({
    id: libraryItem.bookId,
    title: libraryItem.title,
  return book


new BestMatchLoader<KeyType, ObjectType>({
  // see PrimaryKeyLoader options for discussion of context parameter
  fetch: async (keys: KeyType[], context) => ObjectType[] // required

  // take a key and an object and determine how well they match
  // each key will only pick one "winner" which will be the item that returns
  // the highest score
  // return a score of 0 when the key and item do not match at all
  scoreMatch: (key: KeyType, item: ObjectType) => number

  // specify one or more primary key loaders and they will automatically
  // be primed with any results gathered
  // NOTE: if the above fetch function returns result objects that differ from those of
  // the specified loader(s), it's going to cause you problems
  idLoader: PrimaryKeyLoader|PrimaryKeyLoader[],

  // the options object to pass to dataloader upon creation
  // see dataloader documentation for details
  // the default batch size will be 100 here, to help limit the impact of the O(n^2)
  // nature of the BestMatchLoader's scoring strategy
  options: DataLoader.Options

Parameter-based filtering

Now we get to the part where this library can really save your bacon. Consider the following GraphQL query:

{ authors { books(genre: "mystery") { title } } }

Without dataloader-factory, the typical pattern for the authors.books dataloader looks like this:

const booksByAuthorId = new DataLoader(async (authorIds) => {
  const books = await db.query(
    `SELECT * FROM books WHERE authorId IN (${ => '?').join(',')})`
  , authorIds)
  const bookMap = lodash.groupBy(books, 'authorId')
  return => bookMap[id] || [])

Easy so far, but adding the genre: "mystery" filter is not obvious and can be very confusing to implement. You can only batch on one key at a time, so how are you supposed to batch when you have both an authorId and a genre to filter with?

Since genre: "mystery" is the argument, and authors is the array in this query, clearly we want to batch on authorId. But if I call .load(authorId), how am I going to pass the genre to the batch function? I could try something like .load({ authorId, genre }), but transforming that into SQL is going to be a nightmare as the number of filters gets larger.

The answer is actually to create a new DataLoader instance for each unique set of extra filtering arguments. So in the genre case, we need a DataLoader for mysteries, another DataLoader for fantasy, and so on. Since we don't want to hard-code an exhaustive list of genres, we need a function that can generate new DataLoaders, and since we don't want to use infinite RAM, it should do so on demand. Well we're in luck, because that's exactly what dataloader-factory was built to do!

So using dataloader-factory, it becomes fairly simple; the resolver would look like this (ignore the overly simplistic data model):

import { OneToManyLoader } from 'dataloader-factory'
const booksByAuthorIdLoader = new OneToManyLoader({
  fetch: (authorIds, filters) => {
    const query = `SELECT * FROM books WHERE authorId IN (${'?').join(',')})`
    const params = [...authorIds]
    if (filters.genre) {
      query += ' AND genre=?'
    return db.query(query, params)
  extractKey: book => book.authorId
export const authorBooksResolver = (author, filters, context) => {
  return context.dataLoaderFactory.get(booksByAuthorIdLoader, filters).load(

Note that the arguments we get from GraphQL get passed to the .get() function, NOT the .load() function. The .load() function belongs to DataLoader. I don't change it in any way. The .get() function stringifies the filters and uses that as a key to determine whether a new DataLoader should be constructed. Then the filters get passed (unstringified) to the fetch function so it can use them to do its work.

What this does is generate a distinct DataLoader instance for each set of GraphQL arguments we encounter. Generating so many DataLoaders may seem like a problem, but remember - graphql queries are always finite, so there can only be a few sets of arguments in any one request, and each request gets a new factory. So the number of possible dataloaders generated per request is manageable, and they all get garbage collected after the request.

Advanced Usage Example

Many GraphQL data types will have more than one other type referencing them. In those cases, it will probably be useful to create a single function that accepts a set of filters and uses it to construct and execute a database query. Then each fetch function can simply merge the batch of keys into the filters object, and pass the merged object to the query function. This example should help illustrate:

const executeBookQuery = filters => {
  const where = []
  const params = []
  if (filters.ids) {
    where.push(`id IN (${ => '?').join(',')})`)
  if (filters.authorIds) {
    where.push(`authorId IN (${ => '?').join(',')})`)
  if (filters.genres) {
    where.push(`genres IN (${ => '?').join(',')})`)
  const wherestr = where.length && `WHERE (${where.join(') AND (')})`
  return db.query(`SELECT * FROM books ${wherestr}`, params)
const booksByAuthorIdLoader = new OneToManyLoader({
  fetch: (authorIds, filters) {
    return executeBookQuery({ ...filters, authorIds })
  extractKey: item => item.authorId
const booksByGenreLoader = new OneToManyLoader({
  fetch: (genres, filters) => {
    return executeBookQuery({ ...filters, genres })
  extractKey: item => item.genre

See how executeBookQuery is re-usable no matter how many different types of filtering we add? You don't have to use this pattern but I find it very helpful.

Note about filter and batch overlap

One situation that pops up when you start using this strategy and allowing filters on field resolvers is your users can start writing queries that don't make a lot of sense:

{ genres {
    books (genres: ["mystery", "fantasy"]) {

This doesn't make a lot of sense because you're going to get results like:

  name: 'nonfiction',
  books: [ /* only books tagged with nonfiction AND mystery or fantasy */ ]
  name: 'mystery',
  books: [ /* all the books you'd expect */ ]
}, /* etc */]

Even then, making it work that way is complicated. It translates to SQL like genre IN (... batchedGenres ...) AND genre IN ('mystery','fantasy'), which can be a pain to try to generate.

dataloader-factory has one more trick up its sleeve for this if you opt to use it. Simply provide a keysFromFilter function that takes your filters object and returns the key or keys exactly as extractKey would return the key from the model:

const booksByGenreLoader = new ManyToManyLoader({
  fetch: async (genres, filters) => await executeBookQuery({ ...filters, genres }),
  extractKeys: book => book.genres,
  keysFromFilter: filters => filters.genres

When you provide keysFromFilter, dataloader-factory will perform application-side filtering to remove any objects that don't match your filter (it uses the cacheKeyFn to compare them). Your database query need only worry about the batch keys.

Note that in my fetch function above I write executeBookQuery({ ...filters, genres }). Order is important in that merge operation, as the batch keys MUST overwrite the corresponding filter parameter, not the other way around.

Compound Keys

Compound Keys are fully supported. Any key object will be accepted. It is up to your fetch and extractKey functions to treat it properly. Internally, a stable JSON stringify is used to cache results, so it will construct the same string even if two objects' keys have mismatching ordering.


In rare cases it may be that you are unable to provide an extractKey function because a key cannot be extracted from an item because an irreversible operation is involved (like evaluating greater than or less than)

In those cases, you can provide a matchKey function that examines whether the result object is a match for the given key. The answer will help us put the fetched dataset back together properly.

Please note that this makes the batched load an O(n^2) operation so extractKey is preferred whenever possible. If you use matchKey, the maxBatchSize default will change to 50 to help limit scaling problems.

const booksAfterYearLoader = new OneToManyLoader({
  fetch: (years, filters) => {
    const ors = => `published > DATE('${year}0101')`)
    return db.query(`SELECT * FROM books WHERE ${ors.join(') OR (')}`
  matchKey: (year, book) => book.published.getTime() >= new Date(year, 0, 1)


Dataloader has a .loadMany method that can be used to retrieve objects based on an array of keys. However, it is designed to catch and return Errors for any keys that throw an error, so you have to be aware of that any time you use it. Additionally, keys that do not point at any data will come back undefined, and you will have undefined values in the returned array.

To avoid both of these problems, a .loadMany method exists on the factory for your convenience. Its return array only contains objects that exist - no undefined values. Additionally, if any key throws an error, loadMany throws the error instead of catching it and inserting it into the array.

const books = await ctx.loaders.loadMany(bookLoader, bookIds)

It also works for *ToMany loaders and flattens the results (this would return an array of books written by any of the specified authors):

const books = await ctx.loaders.loadMany(booksByAuthorLoader, authorIds, filters)


In graphql, mutations have return values, and the user is able to query any number and depth of properties in that return object. In effect the user has a chance to make a new query after the mutation, and you may have already done some dataloading (e.g. to find related objects to help authorize the mutation). You will want to get rid of any cached objects that were fetched before the mutation completed. Creating a new DataLoaderFactory instance is one way, but probably cumbersome. Instead of replacing your instance, you can call .clear() on your instance and all your existing dataloaders will be tossed so that your post-mutation query can run against fresh data.


This library is written in typescript and provides its own types. When you create a new loader type, you can choose whether to provide your types as generics, which will help you write your fetch function properly, or you can write your fetch function and its input/return types will be used implicitly for everything else.

import { PrimaryKeyLoader } from 'dataloader-factory'
const bookLoader = new PrimaryKeyLoader({
  fetch: async (ids: string[]) => { // provide the key type either here or as a generic
    ... // return type will infer based on what you return here, or you can set it as a generic
  extractId: (item) => { // typescript should know you'll receive IBook
    ... // typescript should know you need to return string

export const bookResolver = async (book, args, context) => {
  // typescript should know load() accepts a string
  // and that bookResolver will return Promise<IBook>
  return await context.dataLoaderFactory.get(bookLoader).load(book.authorId)

The *ToMany classes work the same way, with a third generic for FilterType:

const booksByAuthorIdLoader = new OneToManyLoader({
  fetch: (authorIds: string[], filters: BookFilters) {
    // typescript will detect ReturnType = IBook if this returns Promise<IBook[]>
    return executeBookQuery({ ...filters, authorIds })
  extractKey: item => item.authorId
export const authorBooksResolver = (author, args, context) => {
  // typescript should know load() accepts a string
  // and that authorBooksResolver will return Promise<IBook[]>
  return context.dataLoaderFactory.get(authorBooksLoader, args).load(

Upgrading from 3.0

The 4.0 release is a major API revision that focuses on the typescript-safe API outlined in this README. The old string-based .register and .get and .getOneToMany and etc are all gone. You'll need to update your code to change over to the new API, but the configuration options have not changed (except matchKey has been removed from the ManyJoinedLoader since it doesn't make sense).

DataLoaderFactory.register('youruniquestring', { /* your config */ })
// inside your resolvers

should be replaced with

const myLoader = new PrimaryKeyLoader({ /* your config */ })
// inside your resolvers

The transition is very similar for the array-returning types, except all the .getOneToMany, .getManyToMany, etc, have been removed since .get() is simpler and can cover everything in 4.0.

If you were using typesafe classes already

If you were aready on the typesafe API, all you need to handle is that factory.getMany has been replaced in all cases by factory.get.

factory.getMany(myOneToManyLoader, args).load(id)

should be replaced with

factory.get(myOneToManyLoader, args).load(id)


Package last updated on 22 Jul 2023

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