Dataloader Plugin

This plugin makes it easy to add fields and types that are loaded through a dataloader.

Usage

$3

To use the dataloader plugin you will need to install both the dataloader package and the Pothos
dataloader plugin:

``bash
yarn add dataloader @pothos/plugin-dataloader
`

$3

`typescript
import DataloaderPlugin from '@pothos/plugin-dataloader';

const builder = new SchemaBuilder({
plugins: [DataloaderPlugin],
});
`

$3

To create an object type that can be loaded with a dataloader use the new builder.loadableObject
method:

`typescript
const User = builder.loadableObject('User', {
// load will be called with ids of users that need to be loaded
// Note that the types for keys (and context if present) are required
load: (ids: string[], context: ContextType) => context.loadUsersById(ids),
fields: (t) => ({
id: t.exposeID('id', {}),
username: t.string({
// the shape of parent will be inferred from loadUsersById() above
resolve: (parent) => parent.username,
}),
}),
});
`

It is VERY IMPORTANT to return values from load in an order that exactly matches the order of
the requested IDs. The order is used to map results to their IDs, and if the results are returned in
a different order, your GraphQL requests will end up with the wrong data. Correctly sorting results
returned from a database or other data source can be tricky, so there this plugin has a sort
option (described below) to simplify the sorting process. For more details on how the load function
works, see the dataloader docs.

When defining fields that return Users, you will now be able to return either a string (based in
ids param of load), or a User object (type based on the return type of loadUsersById).

`typescript
builder.queryType({
fields: (t) => ({
user: t.field({
type: User,
args: {
id: t.arg.string({ required: true }),
},
// Here we can just return the ID directly rather than loading the user ourselves
resolve: (root, args) => args.id,
}),
currentUser: t.field({
type: User,
// If we already have the user, we use it, and the dataloader will not be called
resolve: (root, args, context) => context.currentUser,
}),
users: t.field({
type: [User],
args: {
ids: t.arg.stringList({ required: true }),
},
// Mixing ids and user objects also works
resolve: (_root, args, context) => [...args.ids, context.CurrentUser],
}),
}),
});
`

Pothos will detect when a resolver returns string, number, or bigint (typescript will
constrain the allowed types to whatever is expected by the load function). If a resolver returns an
object instead, Pothos knows it can skip the dataloader for that object.

$3

In some cases you may need more granular dataloaders. To handle these cases there is a new
t.loadable method for defining fields with their own dataloaders.

`typescript
// Normal object that the fields below will load
interface PostShape {
id: string;
title: string;
content: string;
}

const Post = builder.objectRef('Post').implement({
fields: (t) => ({
id: t.exposeID('id', {}),
title: t.exposeString('title', {}),
content: t.exposeString('title', {}),
}),
});

// Loading a single Post
builder.objectField(User, 'latestPost', (t) =>
t.loadable({
type: Post,
// will be called with ids of latest posts for all users in query
load: (ids: number[], context) => context.loadPosts(ids),
resolve: (user, args) => user.lastPostID,
}),
);
// Loading multiple Posts
builder.objectField(User, 'posts', (t) =>
t.loadable({
type: [Post],
// will be called with ids of posts loaded for all users in query
load: (ids: number[], context) => context.loadPosts(ids),
resolve: (user, args) => user.postIDs,
}),
);
`

$3

loadable fields can return lists, but do not work for loading a list of records from a single id.

The loadableList method can be used to define loadable fields that represent this kind of
relationship.

`typescript
// Loading multiple Posts
builder.objectField(User, 'posts', (t) =>
t.loadableList({
// type is singular, but will create a list field
type: Post,
// will be called with ids of all the users, and should return Post[][]
load: (ids: number[], context) => context.postsByUserIds(ids),
resolve: (user, args) => user.id,
}),
);
`

$3

You can provide additional options for your dataloaders using loaderOptions.

`typescript
const User = builder.loadableObject('User', {
loaderOptions: { maxBatchSize: 20 },
load: (ids: string[], context: ContextType) => context.loadUsersById(ids),
fields: (t) => ({ id: t.exposeID('id', {}) }),
});

builder.objectField(User, 'posts', (t) =>
t.loadable({
type: [Post],
loaderOptions: { maxBatchSize: 20 },
load: (ids: number[], context) => context.loadPosts(ids),
resolve: (user, args) => user.postIDs,
}),
);
`

See dataloader docs for all available options.

$3

Dataloaders for "loadable" objects can be accessed via their ref by passing in the context object
for the current request. dataloaders are not shared across requests, so we need the context to get
the correct dataloader for the current request:

`typescript
// create loadable object
const User = builder.loadableObject('User', {
load: (ids: string[], context: ContextType) => context.loadUsersById(ids),
fields: (t) => ({
id: t.exposeID('id', {}),
}),
});

builder.queryField('user', (t) =>
t.field({
type: User,
resolve: (parent, args, context) => {
// get data loader for User type
const loader = User.getDataloader(context);

// manually load a user
return loader.load('123');
},
}),
);
`

$3

Calling dataloader.loadMany will resolve to a value like (Type | Error)[]. Your load function
may also return results in that format if your loader can have parital failures. GraphQL does not
have special handling for Error objects. Instead Pothos will map these results to something like
(Type | Promise)[] where Errors are replaced with promises that will be rejected. This
allows the normal graphql resolver flow to correctly handle these errors.

If you are using the loadMany method from a dataloader manually, you can apply the same mapping
using the rejectErrors helper:

`typescript
import { rejectErrors } from '@pothos/plugin-dataloader';

builder.queryField('user', (t) =>
t.field({
type: [User],
resolve: (parent, args, context) => {
const loader = User.getDataloader(context);

return rejectErrors(loader.loadMany(['123', '456']));
},
}),
);
`

$3

If you want to make dataloaders accessible via the context object directly, there is some additional
setup required. Below are a few options for different ways you can load data from the context
object. You can determine which of these options works best for you or add you own helpers.

First you'll need to update the types for your context type:

`typescript
import { LoadableRef } from '@pothos/plugin-dataloader';

export interface ContextType {
userLoader: DataLoader; // expose a specific loader
getLoader: (ref: LoadableRef) => DataLoader; // helper to get a loader from a ref
load: (ref: LoadableRef, id: K) => Promise; // helper for loading a single resource
loadMany: (ref: LoadableRef, ids: K[]) => Promise<(Error | V)[]>; // helper for loading many
// other context fields
}
`

next you'll need to update your context factory function. The exact format of this depends on what
graphql server implementation you are using.

`typescript
import { initContextCache } from '@pothos/core';
import { LoadableRef, rejectErrors } from '@pothos/plugin-dataloader';

export const createContext = (req, res): ContextType => ({
// Adding this will prevent any issues if you server implementation
// copies or extends the context object before passing it to your resolvers
...initContextCache(),

// using getters allows us to access the context object using this
get userLoader() {
return User.getDataloader(this);
},
get getLoader() {
return (ref: LoadableRef) => ref.getDataloader(this);
},
get load() {
return (ref: LoadableRef, id: K) => ref.getDataloader(this).load(id);
},
get loadMany() {
return (ref: LoadableRef, ids: K[]) =>
rejectErrors(ref.getDataloader(this).loadMany(ids));
},
});
`

Now you can use these helpers from your context object:

`typescript
builder.queryFields((t) => ({
fromContext1: t.field({
type: User,
resolve: (root, args, { userLoader }) => userLoader.load('123'),
}),
fromContext2: t.field({
type: User,
resolve: (root, args, { getLoader }) => getLoader(User).load('456'),
}),
fromContext3: t.field({
type: User,
resolve: (root, args, { load }) => load(User, '789'),
}),
fromContext4: t.field({
type: [User],
resolve: (root, args, { loadMany }) => loadMany(User, ['123', '456']),
}),
}));
`

$3

If you are using the Relay plugin, there is an additional method loadableNode that gets added to
the builder. You can use this method to create node objects that work like other loadeble objects.

`typescript
const UserNode = builder.loadableNode('UserNode', {
id: {
resolve: (user) => user.id,
},
load: (ids: string[], context: ContextType) => context.loadUsersById(ids),
fields: (t) => ({}),
});
`

$3

You may run into type errors if you define 2 loadable objects that circularly reference each other
in their definitions.

There are a some general strategies to avoid this outlined in the
circular-references guide.

This plug also has methods for creating refs (similar to builder.objectRef) that can be used to
split the definition and implementation of your types to avoid any issues with circular references.

`typescript
const User = builder.loadableObjectRef('User', {
load: (ids: string[], context: ContextType) => context.loadUsersById(ids),
});

User.implement({
fields: (t) => ({
id: t.exposeID('id', {}),
}),
});

// Or with relay
const UserNode = builder.loadableNodeRef('UserNode', {
load: (ids: string[], context: ContextType) => context.loadUsersById(ids),
id: {
resolve: (user) => user.id,
},
});

UserNode.implement({
isTypeOf: (obj) => obj instanceof User,
fields: (t) => ({}),
});
`

All the plugin specific options should be passed when defining the ref. This allows the ref to be
used by any method that accepts a ref to implement an object:

`typescript
const User = builder.loadableObjectRef('User', {
load: (ids: string[], context: ContextType) => context.loadUsersById(ids),
});

builder.objectType(User, {
fields: (t) => ({
id: t.exposeID('id', {}),
}),
});
`

The above example is not useful on its own, but this pattern will allow these refs to be used with
other that also allow you to define object types with additional behaviors.

$3

When manually loading a resource in a resolver it is not automatically added to the dataloader
cache. If you want any resolved value to be stored in the cache in case it is used somewhere else in
the query you can use the cacheResolved option.

The cacheResolved option takes a function that converts the loaded object into it's cache Key:

`typescript
const User = builder.loadableObject('User', {
load: (ids: string[], context: ContextType) => context.loadUsersById(ids),
cacheResolved: user => user.id,
fields: (t) => ({
id: t.exposeID('id', {}),
...
}),
});
`

Whenever a resolver returns a User or list or Users, those objects will automatically be added the
dataloaders cache, so they can be re-used in other parts of the query.

$3

As mentioned above, the load function must return results in the same order as the provided array
of IDs. Doing this correctly can be a little complicated, so this plugin includes an alternative.
For any type or field that creates a dataloader, you can also provide a sort option which will
correctly map your results into the correct order based on their ids. To do this, you will need to
provide a function that accepts a result object, and returns its id.

`typescript
const User = builder.loadableObject('User', {
load: (ids: string[], context: ContextType) => context.loadUsersById(ids),
sort: user => user.id,
fields: (t) => ({
id: t.exposeID('id', {}),
...
}),
});
`

This will also work with loadable nodes, interfaces, unions, or fields.

When sorting, if the list of results contains an Error the error is thrown because it can not be
mapped to the correct location. This sort option should NOT be used for cases where the result
list is expected to contain errors.

$3

Defining multiple functions to extract the key from a loaded object can become redundant. In cases
when you are using both cacheResolved and sort you can use a toKey function instead:

`typescript
const User = builder.loadableObject('User', {
load: (ids: string[], context: ContextType) => context.loadUsersById(ids),
toKey: user => user.id,
cacheResolved: true,
sort: true,
fields: (t) => ({
id: t.exposeID('id', {}),
...
}),
});
``