How to get batched resolvers working

Hello,

I was trying to use the @Batched annotation interface to improve the speed of some of my resolvers but i don't seem to get around the error message that tells me that my source-type isn't an iterable. I have a resolver which has non-batched fields and batched fields. I added the annotation to the method of a batched field and wrapped the original return type in a list. I also wrapped the first argument (source) in a list.

The error occurs on query time:

Exception while fetching data (/getFoos/edges[0]/node/Bar) : Source type (my.package.Foo) is not expected type (java.lang.Iterable)!

The last part of stack trace is:

at com.coxautodev.graphql.tools.MethodFieldResolver$createDataFetcher$1.invoke(MethodFieldResolver.kt:57) ~[graphql-java-tools-5.6.1.jar:?]
	at com.coxautodev.graphql.tools.MethodFieldResolver$createDataFetcher$1.invoke(MethodFieldResolver.kt:23) ~[graphql-java-tools-5.6.1.jar:?]
	at com.coxautodev.graphql.tools.MethodFieldResolverDataFetcher.get(MethodFieldResolver.kt:160) ~[graphql-java-tools-5.6.1.jar:?]
	at com.coxautodev.graphql.tools.BatchedMethodFieldResolverDataFetcher.get(MethodFieldResolver.kt:198) ~[graphql-java-tools-5.6.1.jar:?]
	at graphql.execution.ExecutionStrategy.fetchField(ExecutionStrategy.java:270) ~[graphql-java-13.0.jar:?]

The original (resolver) code (without batching) was:

public class FooResolver implements GraphQLResolver<Foo> {

    private final BarRetriever barRetriever;

    public int getId(Foo f) {
        return f.getFooId()
    }

    public List<Bar> getBars(Foo foo) {
       return barRetriever.getBarsForFooId(foo.getId)
    }
}

public class FooResolver implements GraphQLResolver<Foo> {
  
    private final BarRetriever barRetriever;

    public int getId(Foo f) {
        return f.getFooId()
    }

    @Batched
    public List<List<Bar>> getBars(List<Foo> foos) {
       Set<Integer> ids = foos.stream().map(Foo::getFooId).collect(Collectors.toList());
       return barRetriever.getBarsForFooIds(ids)
    }
}

The GraphQL schema is:

type Query {
   getFoos: FooConnection @connection(for: "Foo")
}

type Foo {
   id: Int!
   bars: [Bar!]!
}

What am i doing wrong?

[oliemansm]

I think you might want to take a look at using Dataloaders, see https://github.com/graphql-java-kickstart/graphql-java-servlet#context-and-dataloader-settings.

Thank you very much! The missing key was that a method in a resolver is allowed to return an CompletableFuture. Then it is possible to use the dataLoaderRegistry as is designed in graphql java :)

[oliemansm]

Great! So we can close this one now too?

It might be nice to have an example of how to create bataching in graphql-java-tools... but for now it helped me a lot thanx :)