Expected Iterable, but did not find one for field

[BarryThePenguin]

When a schema defines a list using an interface, it looks like @miragejs/graphql fails to build the field for the list

interface ItemRead {
  authors: [String!]!
  id: ID!
  title: String!
}

type BookChapterRead implements ItemRead {
  authors: [String!]!
  chapter: Int
  id: ID!
  title: String!
}

type BookRead implements ItemRead {
  authors: [String!]!
  id: ID!
  rating: Int
  title: String!
}

type Query {
  booksRead: [BookRead]
  bookChapterRead: [BookChapterRead]
  itemsRead: [ItemRead]
}

Querying for..

{
  itemsRead {
    id
    title
    authors
  }
}

..fails with..

Expected Iterable, but did not find one for field "Query.itemsRead".

..while querying other fields works fine.

I put together a reproduction in the REPL https://miragejs.com/repl/v2/042996bcd5

[BarryThePenguin]

Looking at the mirageGraphQLFieldResolver, it may need additional checks for lists that are a GraphQLInterfaceType or GraphQLUnionType?

graphql/lib/resolvers/mirage.js

Lines 29 to 41 in 8364bb6

	export default function mirageGraphQLFieldResolver(obj, args, context, info) {
	let { isList, type } = unwrapType(info.returnType);
	return isInterfaceType(type)
	? resolveInterface(obj, args, context, info, type)
	: isUnionType(type)
	? resolveUnion(obj, args, context, info, isList, type)
	: !isObjectType(type)
	? resolveDefault(obj, args, context, info)
	: isList
	? resolveList(obj, args, context, info, type)
	: resolveObject(obj, args, context, info, type);
	}

[jneurock]

I can't remember the auto-resolution logic at play here but it might be the case that the library will only try to auto-resolve types that implement an interface. I'll have to check as soon as I get a chance. In the meantime, you should be able to add your own resolver for the field, if you haven't done so already.

[BarryThePenguin]

Ok, thanks! I hadn't considered adding a field resolver

[BarryThePenguin]

I didn't have much luck making a field resolver.. But I was able to reproduce the issue in #56

[jneurock]

I think this will work for you https://miragejs.com/repl/v2/571a844c0b:

this.post("/graphql", createGraphQLHandler(graphQLSchema, this.schema, {
  resolvers: {
    Query: {
      itemsRead(_obj, _args, context) {
        const booksRead = context.mirageSchema.db.bookReads.map((book) => ({
          ...book,
          __typename: 'BookRead'
        }));
        const bookChaptersRead = context.mirageSchema.db.bookChapterReads.map((chapter) => ({
          ...chapter,
          __typename: 'BookChapterRead'
        }));
        
        return [...booksRead, ...bookChaptersRead];
      }
    }
  }
}))

Mirage's inflector doesn't seem to be able to handle the pluralization well in this case so the table names are a bit weird.