Support lists in Apollo Federation reference resolvers

[luke-pp]

Product

Hot Chocolate

Version

14.2.0

Link to minimal reproduction

e96c3b7#diff-9ffdbfd9cc96a0b754318fd0f72577c4f85bbc8721d5ade34204c92a0aa07c25R371

Steps to reproduce

1. Create a GraphQL type with a reference resolver using HotChocolate.ApolloFederation
2. Add a field that is a list, and mark it as [External]
3. Add the list field as an argument to the reference resolver
   Example:

public class ListFieldType
    {
        public ListFieldType(string id, List<double> listField)
        {
            Id = id;
            ListField = listField;
        }
        [Key]
        [External]
        public string Id { get; }
        [External]
        public List<double> ListField { get; }
        public string InternalField { get; set; } = "InternalValue";
        [ReferenceResolver]
        public static ListFieldType GetByExternal(string id, List<double> listField, IResolverContext context)
        {
            return new(id, listField);
        }
    }

4. Query the reference resolver, passing in a value for the external field

What is expected?

At this point, I expect to see a value passed into the reference resolver's arguments.

What is actually happening?

The value passed into the reference resolver is always null.

Relevant log output

Additional context

I have created a branch that adds this case to the existing tests dealing with External fields. The root cause seems to be that the ArgumentParser class does not currently handle list types - I have also added the beginnings of an implementation in the branch to confirm that adding that missing logic does fix the issue.

While I don't see any explicit confirmation in the Apollo documentation that external list fields are supported, it does seem like something that should work.

I'm happy to create a PR based on the branch linked above if we agree that this functionality is something worth supporting - it'll need a bit of tidying up as there are some edge cases I haven't fully implemented handling for yet, and the way reflection is being used is a bit messy.

[michaelstaib]

Create a PR and we will see what needs to be done.