Paging of batch-loaded, nested fields in Spring GraphQL

Question

I have the following schema structure (albeit with entity names changed):

type Artist {
 name: String!
 fans(first: Int, after: String, last: Int, before: String): FanConnection!
}
type ArtistConnection {
 edges: [ArtistEdge!]!
 pageInfo: PageInfo!
}
type ArtistEdge {
 cursor: String!
 node: Artist!
}
type FanConnection {
 edges: [FanEdge!]!
 pageInfo: PageInfo!
}
type FanEdge {
 cursor: String!
 node: Fan!
}
type Fan {
 name: String!
}
type PageInfo {
 hasPreviousPage: Boolean!
 hasNextPage: Boolean!
 startCursor: String
 endCursor: String
}
type Query {
 artists(first: Int, after: String, last: Int, before: String): ArtistConnection!
}

My goal is to batch resolve Fan entities when listing artists and their fans. To do this I had been using Spring Data's Window as a return value from data fetchers (controller methods) which automatically is mapped to a GraphQL connection by Spring GraphQL. I am referencing the ScrollSubrange argument provided automatically by Spring GraphQl to my controller methods, therefore, I am unable to use the @BatchMapping annotation; this means I have to register batch data loaders manually, and given all these batch data loaders' values are 'Window.class', I have to register then using forName rather than forTypePair - which then means that I end up not being able to resolve Java type issues where the data loader is expecting Mono<Map<Object, Object>>> but my data loader is returning Mono<Map<Artist, Window<Fan>>>>.

What is the correct way to handle batch loading Fan entities for all Artist entities in the result of this query:

query {
 artists {
 name
 fans(first: 10) {
 name
 }
 }
}

I can't seem to find specific batch mapping + paging examples, or documentation anywhere.

Here's what I had so far:

Registration of the data loader:

 FanController(final FanService fanService, final BatchLoaderRegistry batchLoaderRegistry) {
 this.fanService = fanService;
 batchLoaderRegistry
 .forName("artistFansDataLoader")
 .registerMappedBatchLoader((final Set<Artist> artists, final BatchLoaderEnvironment environment) -> {
 final GraphQLContext graphQLContext = Optional.ofNullable(environment.getContext())
 .map(GraphQLContext.class::cast)
 .orElseThrow(() -> new InternalError("Failed to get GraphQL Context"));
 final Identity identity = Optional.ofNullable(SecurityContextHolder.getContext().getAuthentication())
 .map(Authentication::getPrincipal)
 .map(Identity.class::cast)
 .orElseThrow(() -> new InternalError("Failed to get authentication principal"));
 final ScrollSubrange scrollSubrange = graphQLContext.get("scrollSubrange");
 return (Mono<Map<Object, Object>>) this.fansByArtists(artists, identity, scrollSubrange);
 });
 }

data fetcher / controller method:

 @SchemaMapping(typeName = "Artist", field = "fans")
 public CompletableFuture<Window<Fan>> artistFans(
 final GraphQLContext context,
 final ScrollSubrange scrollSubrange,
 @Qualifier("artistFansDataLoader") final DataLoader<Artist, Window<Fan>> loader,
 final Artist artist
 ) {
 return loader.load(artist);
 }

and finally, the data loader:

private Mono<Map<Artist, Window<Fan>>> fansByArtists(
 final Set<Artist> artists,
 final Identity identity,
 final ScrollSubrange scrollSubrange
) {
 final Map<Artist, Window> result = artists.parallelStream()
 .collect(Collectors.toMap(
 Function.identity(),
 artist -> this.fanService.list(
 new ListFansRequest(
 identity.accountId(),
 Collections.singleton(artist.id()),
 scrollSubrange
 )
 )
 ));
 return Mono.just(result);
}

This is the type error I'm seeing for the return statement in the registration of the batch data loader:

Inconvertible types; cannot cast 'Mono<Map<Artist,Window>>' to 'Mono<Map<Object,Object>>'

Answer 1

I have managed to get this working by providing generic arguments to the forName method of the BatchLoaderRegistry:

batchLoaderRegistry
 .<Artist, Window<Fan>>forName("artistFansLoader")
 .registerMappedBatchLoader(this::fansByArtistIds);

Now, I am able to properly type the data loader method without issue or raw type warnings.