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Relay can be used to read and write local data, and act as a single source of truth for all data in your client application.
The Relay Compiler fully supports client-side extensions of the schema, which allows you to define local fields and types.

Table of Contents:

• Extending the client schema
• Querying local state
• Mutating local state
• Initial local state

Extending the server schema

To extend the server schema, create a new .graphql file inside your --src directory.
Let's call it ./src/clientSchema.graphql.

This schema describes what local data can be queried on the client.
It can even be used to extend an existing server schema.

For example, we can create a new type called Note:

type Note {
  ID: ID!
  title: String
  body: String
}

And then extend the server schema type User, with a list of Note, called notes.

extend type User {
  notes: [Note]
}

Querying local state

Accessing local data is no different from querying your GraphQL server, although you are required to include atleast one server field in the query. The field can be from the server schema, or it can be schema agnostic, like an introspection field (i.e. __typename).

Here, we use a QueryRenderer to get the current User via the viewer field, along with their id, name and the local list of notes.

// Example.js
import React from 'react';
import { QueryRenderer, graphql } from 'react-relay';

const renderQuery = ({error, props}) => {
  if (error) {
    return <div>{error.message}</div>;
  } else if (props) {
    return (
      <div>
        {props.viewer.notes.map(({id, title, body}) => (
          <div key={id}>
            {title}
          </div>
          <div key={id}>
            {body}
          </div>
        ))}
      </div>
    );
  }
  return <div>Loading</div>;
}

const Example = (props) => {
  return (
    <QueryRenderer
      render={renderQuery}
      environment={environment}
      query={graphql`
        query ExampleQuery {
          viewer {
            id
            name
            notes {
              id
              title
              body
            }
          }
        }
      `}
    />
  );
}

Mutating local state

All local data lives in the Relay Store.
Updating local state can be done with any updater function. The commitLocalUpdate function is especially ideal for this, because writes to local state are usually executed outside of a mutation.

To build upon the previous example, let's try creating, updating and deleting a Note from the list of notes on User.

Create

import {commitLocalUpdate} from 'react-relay';

let tempID = 0;

function createUserNote() {
  commitLocalUpdate(environment, store => {
    const user = store.getRoot().getLinkedRecord('viewer');
    const userNoteRecords = user.getLinkedRecords('notes') || [];

    // Create a unique ID.
    const dataID = `client:Note:${tempID++}`;

    //Create a new note record.
    const newNoteRecord = store.create(dataID, 'Note');

    // Add the record to the user's list of notes.
    user.setLinkedRecords([...userNoteRecords, newNoteRecord], 'notes');
  });
}

Note that since this record will be rendered by the ExampleQuery in our QueryRenderer, the QueryRenderer will automatically retain this data so it isn't garbage collected.

If no component is rendering the local data and you want to manually retain it, you can do so by calling environment.retain():

// Tell Relay to retain the record so it isn't garbage collected
environment.retain({
  dataID,
  variables: {},
  node: { selections: [] }
});

Update

import {commitLocalUpdate} from 'react-relay';

function updateUserNote(dataID, body, title) {
  commitLocalUpdate(environment, store => {
    const note = store.get(dataID);

    note.setValue(body, 'body');
    note.setValue(title, 'title')
  });
}

Delete

import {commitLocalUpdate} from 'react-relay';

function deleteUserNote(dataID) {
  commitLocalUpdate(environment, store => {
    const user = store.getRoot().getLinkedRecord('viewer');
    const userNoteRecords = user.getLinkedRecords('notes');

    // Remove the note from the list of user notes.
    const newUserNoteRecords = userNoteRecords.filter(x => x.getDataID() !== dataID);

    // Delete the note from the store.
    store.delete(dataID);

    // Set the new list of notes.
    user.setLinkedRecords(newUserNoteRecords, 'notes');
  });
}

Initial local state

All new client-side schema fields default to undefined value. Often however, you will want to set the initial state before querying local data. You can use an updater function via commitLocalUpdate to prime local state.

import {commitLocalUpdate} from 'react-relay';

commitLocalUpdate(environment, store => {
  const user = store.getRoot().getLinkedRecord('viewer');

  // initialize user notes to an empty array.
  user.setLinkedRecords([], 'notes');
});