graphql-relay/README.md

# Relay Library for GraphQL.js

This is a library to allow the easy creation of Relay-compliant servers using the [GraphQL.js](https://github.com/graphql/graphql-js) reference implementation of a GraphQL server.

[![Build Status](https://github.com/graphql/graphql-relay-js/workflows/CI/badge.svg?branch=main)](https://github.com/graphql/graphql-relay-js/actions?query=branch%3Amain)
[![Coverage Status](https://codecov.io/gh/graphql/graphql-relay-js/branch/master/graph/badge.svg)](https://codecov.io/gh/graphql/graphql-relay-js)

## Getting Started

A basic understanding of GraphQL and of the GraphQL.js implementation is needed to provide context for this library.

An overview of GraphQL in general is available in the [README](https://github.com/graphql/graphql-spec/blob/master/README.md) for the [Specification for GraphQL](https://github.com/graphql/graphql-spec).

This library is designed to work with the [GraphQL.js](https://github.com/graphql/graphql-js) reference implementation of a GraphQL server.

An overview of the functionality that a Relay-compliant GraphQL server should provide is in the [GraphQL Relay Specification](https://relay.dev/docs/guides/graphql-server-specification/) on the [Relay website](https://relay.dev/). That overview describes a simple set of examples that exist as [tests](src/__tests__) in this repository. A good way to get started with this repository is to walk through that documentation and the corresponding tests in this library together.

## Using Relay Library for GraphQL.js

Install Relay Library for GraphQL.js

```sh
npm install graphql graphql-relay
```

When building a schema for [GraphQL.js](https://github.com/graphql/graphql-js), the provided library functions can be used to simplify the creation of Relay patterns.

### Connections

Helper functions are provided for both building the GraphQL types for connections and for implementing the `resolve` method for fields returning those types.

- `connectionArgs` returns the arguments that fields should provide when they return a connection type that supports bidirectional pagination.
- `forwardConnectionArgs` returns the arguments that fields should provide when they return a connection type that only supports forward pagination.
- `backwardConnectionArgs` returns the arguments that fields should provide when they return a connection type that only supports backward pagination.
- `connectionDefinitions` returns a `connectionType` and its associated `edgeType`, given a node type.
- `connectionFromArray` is a helper method that takes an array and the arguments from `connectionArgs`, does pagination and filtering, and returns an object in the shape expected by a `connectionType`'s `resolve` function.
- `connectionFromPromisedArray` is similar to `connectionFromArray`, but it takes a promise that resolves to an array, and returns a promise that resolves to the expected shape by `connectionType`.
- `cursorForObjectInConnection` is a helper method that takes an array and a member object, and returns a cursor for use in the mutation payload.
- `offsetToCursor` takes the index of a member object in an array and returns an opaque cursor for use in the mutation payload.
- `cursorToOffset` takes an opaque cursor (created with `offsetToCursor`) and returns the corresponding array index.

An example usage of these methods from the [test schema](src/__tests__/starWarsSchema.ts):

```js
var { connectionType: ShipConnection } = connectionDefinitions({
  nodeType: shipType,
});
var factionType = new GraphQLObjectType({
  name: 'Faction',
  fields: () => ({
    ships: {
      type: ShipConnection,
      args: connectionArgs,
      resolve: (faction, args) =>
        connectionFromArray(
          faction.ships.map((id) => data.Ship[id]),
          args,
        ),
    },
  }),
});
```

This shows adding a `ships` field to the `Faction` object that is a connection. It uses `connectionDefinitions({nodeType: shipType})` to create the connection type, adds `connectionArgs` as arguments on this function, and then implements the resolve function by passing the array of ships and the arguments to `connectionFromArray`.

### Object Identification

Helper functions are provided for both building the GraphQL types for nodes and for implementing global IDs around local IDs.

- `nodeDefinitions` returns the `Node` interface that objects can implement, and returns the `node` root field to include on the query type. To implement this, it takes a function to resolve an ID to an object, and to determine the type of a given object.
- `toGlobalId` takes a type name and an ID specific to that type name, and returns a "global ID" that is unique among all types.
- `fromGlobalId` takes the "global ID" created by `toGlobalID`, and returns the type name and ID used to create it.
- `globalIdField` creates the configuration for an `id` field on a node.
- `pluralIdentifyingRootField` creates a field that accepts a list of non-ID identifiers (like a username) and maps them to their corresponding objects.

An example usage of these methods from the [test schema](src/__tests__/starWarsSchema.ts):

```js
var { nodeInterface, nodeField } = nodeDefinitions(
  (globalId) => {
    var { type, id } = fromGlobalId(globalId);
    return data[type][id];
  },
  (obj) => {
    return obj.ships ? factionType : shipType;
  },
);

var factionType = new GraphQLObjectType({
  name: 'Faction',
  fields: () => ({
    id: globalIdField(),
  }),
  interfaces: [nodeInterface],
});

var queryType = new GraphQLObjectType({
  name: 'Query',
  fields: () => ({
    node: nodeField,
  }),
});
```

This uses `nodeDefinitions` to construct the `Node` interface and the `node` field; it uses `fromGlobalId` to resolve the IDs passed in the implementation of the function mapping ID to object. It then uses the `globalIdField` method to create the `id` field on `Faction`, which also ensures implements the `nodeInterface`. Finally, it adds the `node` field to the query type, using the `nodeField` returned by `nodeDefinitions`.

### Mutations

A helper function is provided for building mutations with single inputs and client mutation IDs.

- `mutationWithClientMutationId` takes a name, input fields, output fields, and a mutation method to map from the input fields to the output fields, performing the mutation along the way. It then creates and returns a field configuration that can be used as a top-level field on the mutation type.

An example usage of these methods from the [test schema](src/__tests__/starWarsSchema.ts):

```js
var shipMutation = mutationWithClientMutationId({
  name: 'IntroduceShip',
  inputFields: {
    shipName: {
      type: new GraphQLNonNull(GraphQLString),
    },
    factionId: {
      type: new GraphQLNonNull(GraphQLID),
    },
  },
  outputFields: {
    ship: {
      type: shipType,
      resolve: (payload) => data['Ship'][payload.shipId],
    },
    faction: {
      type: factionType,
      resolve: (payload) => data['Faction'][payload.factionId],
    },
  },
  mutateAndGetPayload: ({ shipName, factionId }) => {
    var newShip = {
      id: getNewShipId(),
      name: shipName,
    };
    data.Ship[newShip.id] = newShip;
    data.Faction[factionId].ships.push(newShip.id);
    return {
      shipId: newShip.id,
      factionId: factionId,
    };
  },
});

var mutationType = new GraphQLObjectType({
  name: 'Mutation',
  fields: () => ({
    introduceShip: shipMutation,
  }),
});
```

This code creates a mutation named `IntroduceShip`, which takes a faction ID and a ship name as input. It outputs the `Faction` and the `Ship` in question. `mutateAndGetPayload` then gets an object with a property for each input field, performs the mutation by constructing the new ship, then returns an object that will be resolved by the output fields.

Our mutation type then creates the `introduceShip` field using the return value of `mutationWithClientMutationId`.

## Contributing

After cloning this repo, ensure dependencies are installed by running:

```sh
npm install
```

This library is written in ES6 and uses [Babel](https://babeljs.io/) for ES5 transpilation and [TypeScript](https://www.typescriptlang.org/) for type safety. Widely consumable JavaScript can be produced by running:

```sh
npm run build
```

Once `npm run build` has run, you may `import` or `require()` directly from node.

After developing, the full test suite can be evaluated by running:

```sh
npm test
```

## Opening a PR

We actively welcome pull requests. Learn how to [contribute](./.github/CONTRIBUTING.md).

This repository is managed by EasyCLA. Project participants must sign the free ([GraphQL Specification Membership agreement](https://preview-spec-membership.graphql.org) before making a contribution. You only need to do this one time, and it can be signed by [individual contributors](https://individual-spec-membership.graphql.org/) or their [employers](https://corporate-spec-membership.graphql.org/).

To initiate the signature process please open a PR against this repo. The EasyCLA bot will block the merge if we still need a membership agreement from you.

You can find [detailed information here](https://github.com/graphql/graphql-wg/tree/main/membership). If you have issues, please email [operations@graphql.org](mailto:operations@graphql.org).

If your company benefits from GraphQL and you would like to provide essential financial support for the systems and people that power our community, please also consider membership in the [GraphQL Foundation](https://foundation.graphql.org/join).

## Changelog

Changes are tracked as [GitHub releases](https://github.com/graphql/graphql-js/releases).

## License

graphql-relay-js is [MIT licensed](./LICENSE).