Subscriptions

Subscription is just another GraphQL operation type like Query and Mutation. It allows creating real-time subscriptions over a bidirectional transport layer, mainly over websockets. Read more about the subscriptions here.

Below is a commentAdded subscription example, copied & pasted directly from the official Apollo documentation:

Subscription: {
  commentAdded: {
    subscribe: () => pubsub.asyncIterator('commentAdded')
  }
}

Notice The pubsub is an instance of PubSub class. Read more about it here.

In order to create an equivalent subscription in Nest-way, we'll make use of the @Subscription() decorator. Let's extend our AuthorResolver used in the resolvers map section.

import { Query, Resolver, Subscription, ResolveProperty } from '@nestjs/graphql';
import { find, filter } from 'lodash';
import { PubSub } from 'graphql-subscriptions';

// example data
const authors = [
  { id: 1, firstName: 'Tom', lastName: 'Coleman' },
  { id: 2, firstName: 'Sashko', lastName: 'Stubailo' },
  { id: 3, firstName: 'Mikhail', lastName: 'Novikov' },
];
const posts = [
  { id: 1, authorId: 1, title: 'Introduction to GraphQL', votes: 2 },
  { id: 2, authorId: 2, title: 'Welcome to Meteor', votes: 3 },
  { id: 3, authorId: 2, title: 'Advanced GraphQL', votes: 1 },
  { id: 4, authorId: 3, title: 'Launchpad is Cool', votes: 7 },
];

// example pubsub
const pubsub = new PubSub();

@Resolver('Author')
export class AuthorResolver {
  @Query('author')
  getAuthor(obj, args, context, info) {
    return find(authors, { id: args.id });
  }

  @Subscription()
  commentAdded() {
    return {
      subscribe: () => pubsub.asyncIterator('commentAdded'),
    };
  }

  @ResolveProperty('posts')
  getPosts(author) {
    return filter(posts, { authorId: author.id });
  }
}

Refactor

We have used a local PubSub instance here. Instead, we should define PubSub as a component, inject it through the constructor (using @Inject() decorator), and reuse it across the entire application. You can read more about Nest custom components here.

Type definitions

The last step is to update type definitions (read more) file.

type Author {
  id: Int!
  firstName: String
  lastName: String
  posts: [Post]
}

type Post {
  id: Int!
  title: String
  votes: Int
}

type Query {
  author(id: Int!): Author
}

type Comment {
  id: String
  content: String
}

type Subscription {
  commentAdded(repoFullName: String!): Comment
}

That's all. We created a single commentAdded(repoFullName: String!): Comment subscription.