Add cloud code resolver example (#771)

* Add cloud code resolver example

* Move cloud resolver code to customisation section

* Add custom schema setup guide

* Modify code example to use graphql-relay instead of Buffer

* Improve custom schema and cloud code resolver docs

_includes/graphql/customisation.md

@@ -287,3 +287,148 @@ You can optionally override the default generated mutation names with aliases:
   ]
 }
 ```
+
+## Cloud Code Resolvers
+
+The Parse GraphQL API supports the use of custom user-defined schema. The [Adding Custom Schema](#adding-custom-schema) section explains how to get started using this feature.
+
+Cloud Code functions can then be used as custom resolvers for your user-defined schema.
+
+### Query Resolvers
+
+Here's an example of a custom query and its related cloud code function resolver in action:
+
+```graphql
+# schema.graphql
+extend type Query {
+  hello: String! @resolve
+}
+```
+
+```js
+// main.js
+Parse.Cloud.define("hello", () => "Hello, world!");
+```
+
+```js
+// Header
+{
+  "X-Parse-Application-Id": "APPLICATION_ID",
+  "X-Parse-Master-Key": "MASTER_KEY" // (optional)
+}
+```
+
+```graphql
+query hello {
+  hello
+}
+```
+
+The code above should resolve to this:
+
+```js
+// Response
+{
+  "data": {
+    "hello": "Hello, world!"
+  }
+}
+```
+
+### Mutation Resolvers
+
+At times, you may need more control over how your mutations modify data than what Parse's auto-generated mutations can provide. For example, if you have classes named `Item` and `CartItem` in the schema, you can create an `addToCart` custom mutation that tests whether a specific item is already in the user's cart. If found, the cart item's quantity is incremented by one. If not, a new `CartItem` object is created.
+
+The ability to branch your resolver logic enables you to replicate functionality found in Parse's auto-generated `createCartItem` and `updateCartItem` mutations and combine those behaviors into a single custom resolver.
+
+```graphql
+# schema.graphql
+extend type Mutation {
+  addToCart(id: ID!): CartItem! @resolve
+}
+```
+
+**Note**: The `id` passed in to your Cloud Code function from a GraphQL query is a [Relay Global Object Identification](https://facebook.github.io/relay/graphql/objectidentification.htm); it is **not** a Parse `objectId`. Most of the time the `Relay Node Id` is a `Base64` of the `ParseClass` and the `objectId`. Cloud code does not recognize a `Relay Node Id`, so converting it to a Parse `objectId` is required.
+
+Decoding and encoding `Relay Node Ids` in Cloud Code is needed in order to smoothly interface with your client-side GraphQL queries and mutations.
+
+First, install the [Relay Library for GraphQL.js](https://www.npmjs.com/package/graphql-relay) as a required dependency to enable decoding and encoding `Relay Node Ids` in your cloud code functions:
+
+```sh
+$ npm install graphql-relay --save
+```
+
+Then, create your `main.js` cloud code file, import `graphql-relay`, and build your `addToCart` function:
+
+```js
+// main.js
+const { fromGlobalId, toGlobalId } = require('graphql-relay');
+
+Parse.Cloud.define("addToCart", async (req) => {
+  const { user, params: { id } } = req;
+
+  // Decode the incoming Relay Node Id to a
+  // Parse objectId for Cloud Code use.
+  const { id: itemObjectId } = fromGlobalId(id);
+
+  // Query the user's current cart.
+  const itemQuery = new Parse.Query("Item");
+  const item = await itemQuery.get(itemObjectId);
+  const cartItemQuery = new Parse.Query("CartItem");
+  cartItemQuery.equalTo("item", item);
+  cartItemQuery.equalTo("user", user);
+  const [existingCartItem] = await cartItemQuery.find();
+  let savedCartItem;
+
+  if (existingCartItem) {
+    // The item is found in the user's cart; increment its quantity.
+    const quantity = await existingCartItem.get("quantity");
+    existingCartItem.set("quantity", quantity + 1);
+    savedCartItem = await existingCartItem.save();
+  } else {
+    // The item has not yet been added; create a new cartItem object.
+    const CartItem = Parse.Object.extend("CartItem");
+    const cartItem = new CartItem();
+    savedCartItem = await cartItem.save({ quantity: 1, item, user });
+  }
+
+  // Encode the Parse objectId to a Relay Node Id
+  // for Parse GraphQL use.
+  const cartItemId = toGlobalId('CartItem', savedCartItem.id);
+
+  // Convert to a JSON object to handle adding the
+  // Relay Node Id property.
+  return { ...savedCartItem.toJSON(), id: cartItemId };
+});
+```
+
+```js
+// Header
+{
+  "X-Parse-Application-Id": "APPLICATION_ID",
+  "X-Parse-Session-Token": "r:b0dfad1eeafa4425d9508f1c0a15c3fa"
+}
+```
+
+```graphql
+mutation addItemToCart {
+  addToCart(id: "SXRlbTpEbDVjZmFWclRI") {
+    id
+    quantity
+  }
+}
+```
+
+The code above should resolve to something similar to this:
+
+```js
+// Response
+{
+  "data": {
+    "addToCart": {
+      "id": "Q2FydEl0ZW06akVVTHlGZnVpQw==",
+      "quantity": 1
+    }
+  }
+}
+```

_includes/graphql/getting-started.md

@@ -93,6 +93,71 @@ After starting the app, you can visit [http://localhost:1337/playground](http://
 
 ⚠️ Please do not mount the GraphQL Playground in production as anyone could access your API Playground and read or change your application's data. [Parse Dashboard](#running-parse-dashboard) has a built-in GraphQL Playground and it is the recommended option for production apps. If you want to secure your API in production take a look at [Class Level Permissions](/js/guide/#class-level-permissions).
 
+## Adding Custom Schema
+
+The Parse GraphQL API supports the use of custom user-defined schema. You can write your own types, queries, and mutations, which will be merged with the ones that are automatically generated. Your custom schema is resolved via [Cloud Code](#cloud-code-resolvers) functions.
+
+First, add a utility for parsing GraphQL queries as a required dependency:
+
+```sh
+$ npm install graphql-tag --save
+```
+
+Then, modify your `index.js` file to include your custom schema, along with the path to your cloud code file:
+
+```js
+const gql = require('graphql-tag');
+
+const parseServer = new ParseServer({
+  appId: 'APPLICATION_ID',
+  cloud: './cloud/main.js',
+});
+
+const parseGraphQLServer = new ParseGraphQLServer(
+  parseServer,
+  {
+    graphQLPath: '/graphql',
+    playgroundPath: '/playground',
+    graphQLCustomTypeDefs: gql`
+      extend type Query {
+        hello: String! @resolve
+        hello2: String! @resolve(to: "hello")
+      }
+    `,
+  }
+);
+```
+
+Alternatively, you can create your custom schema in a dedicated `schema.graphql` file and reference the file in your `index.js`:
+
+```js
+const gql = require('graphql-tag');
+const fs = require('fs');
+const customSchema = fs.readFileSync('./cloud/schema.graphql');
+
+const parseServer = new ParseServer({
+  appId: 'APPLICATION_ID',
+  cloud: './cloud/main.js',
+});
+
+const parseGraphQLServer = new ParseGraphQLServer(
+  parseServer,
+  {
+    graphQLPath: '/graphql',
+    playgroundPath: '/playground',
+    graphQLCustomTypeDefs: gql`${customSchema}`,
+  }
+);
+```
+
+```graphql
+# schema.graphql
+extend type Query {
+  hello: String! @resolve
+  hello2: String! @resolve(to: "hello")
+}
+```
+
 ## Running Parse Dashboard
 
 [Parse Dashboard](https://github.com/parse-community/parse-dashboard) is a standalone dashboard for managing your Parse Server apps, including your objects' schema and data, logs, jobs, CLPs, and push notifications. Parse Dashboard also has a built-in GraphQL Playground that you can use to play around with your auto-generated Parse GraphQL API. It is the recommended option for **production** applications.

_includes/graphql/objects.md

@@ -70,7 +70,7 @@ mutation createAGameScore {
 }
 ```
 
-**Note:** The `id` is a [Relay Global Object Identification](https://facebook.github.io/relay/graphql/objectidentification.htm), it's **not** a Parse `objectId`. Most of the time the `Relay Node Id` is a `Base64` of the `ParseClass` and the `objectId`.
+**Note:** The `id` is a [Relay Global Object Identification](https://facebook.github.io/relay/graphql/objectidentification.htm); it's **not** a Parse `objectId`. Most of the time the `Relay Node Id` is a `Base64` of the `ParseClass` and the `objectId`.
 
 ## Update