How do subscriptions use @Root?

[gblikas]

In https://github.com/MichalLytek/type-graphql/blob/master/examples/redis-subscriptions/recipe.resolver.ts, the newComments method has a @Root parameter,

type-graphql/examples/redis-subscriptions/recipe.resolver.ts

Lines 56 to 71 in 4fc77f5

	@Subscription(returns => Comment, {
	topics: Topic.NewComment,
	filter: ({ payload, args }: ResolverFilterData<NewCommentPayload, NewCommentsArgs>) => {
	return payload.recipeId === args.recipeId;
	},
	})
	newComments(
	@Root() newComment: NewCommentPayload,
	@Args() { recipeId }: NewCommentsArgs,
	): Comment {
	return {
	content: newComment.content,
	date: new Date(newComment.dateString), // limitation of Redis payload serialization
	nickname: newComment.nickname,
	};
	}

I'm having a hard time understanding how a method decorated with @Subscription can have a root object; calling newComments as a subscription is - as in the examples,

type-graphql/examples/redis-subscriptions/examples.gql

Lines 29 to 35 in 4fc77f5

	subscription NewCommentsForRecipe2 {
	newComments(recipeId: "2") {
	nickname
	content
	date
	}
	}

This implies no higher-order object, whereas, using @Root seems to imply that you could do something like

someCommentOnARecipieThing {
  newComments(recipeId: "2") {
      nickname
      content
      date
    }
}

Can I get an example of how type-graphql uses a @Root object in relation to a subscription? Perhaps, how can I re-write my last code-block to be "correct"?

I feel like I'm not understanding the nuance between how type-graphql uses topics, filters and subscriptions, when contrasted against a more-traditional implementation that might use a subscribe: () => pubsub.asyncIterator(["SOME_TOPIC"]) pattern.

In contrast, the @Args() { recipeId }: NewCommentsArgs part makes total sense, in conjunction with the @subscription decorator; you want to subscribe to some recipe's new comments, in context recipeId: "2".

[MichalLytek]

I did that 4 years ago, I wouldn't name that now @Root() 😅
Some GraphQL tools call it parent, source, root - it's basically the first parameter of the resolver.

In case of subscriptions, it's the payload from the pub sub - the published message.