typesafe selectionSets in the presence of filtering

[leemhenson]

Hi.

I have a subschema (outside of my control) that looks like this:

type Author {
  id: ID!
  name: String!
  bookIds: [ID!]!
}

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

and another subschema (outside of my control) that looks like:

type Book {
  id: ID!
  name: String!
}

type Query {
  book(id: ID!): Book 
}

and I want to produce the following stitched schema:

type Book {
  id: ID!
  name: String!
}

type Author {
  id: ID!
  name: String!
  books: [Book!]!  # <- replace bookIds with books
}

type Query {
  author(id: ID!): Author
  book(id: ID!): Book
}

I'm using this .meshrc.yaml:

# ...sources etc

transforms:
    - filterSchema:
          filters:
              - Author.!bookIds

additionalTypeDefs: |
    extend type Author {
      books: [Book!]!
    }

additionalResolvers:
    - ./src/mesh-resolvers.ts

And this additional resolver:

export const resolvers: Resolvers = {
    workflows: {
        selectionSet: '{ bookIds }',
        resolve: async (root, _, context, info) => {
            const rootWithSelectionSet: typeof root & {
                bookIds: string[];
            } = root as any;

            return Promise.all(
                rootWithSelectionSet.bookIds.map(bookId =>
                    context.BooksService.Query.book({
                        root,
                        args: { id: bookId },
                        context,
                        info,
                    })
                )
            );
        },
    },
}

And this works ✅ . However, it's not typesafe as you can see I have to cast the root so it knows I have added bookIds to the selection set. Is there any way to make mesh build typecheck the selectionSet and merge it with the parent type? Thanks!