Wrong/Missing Typescript type for `__resolveReference`

This bug report should include:

• A short, but descriptive title. The title doesn’t need “Apollo” in it.
• The package name and version of Apollo showing the problem.

apollo-graphql, @apollo/federation

• If applicable, the last version of Apollo where the problem did not occur.
• The expected behavior.

__resolveReference can be typed with GraphQLFieldResolver (or another new type?) from the graphql library.

• The actual behavior.

When using Typescript, resolvers passed into buildFederatedSchema can be typed with the GraphQLResolverMap interface defined from apollo-graphql.

e.g.

import { GraphQLResolverMap } from 'apollo-graphql';

/**
 * service graphql resolvers.
 */
const resolvers: GraphQLResolverMap<Context> = {
  Query,
  Mutation,
  Foo,
  DateTime,
};

When looking at the type for GraphQLResolverMap you can see that resolvers can be typed with GraphQLFieldResolver. This type does not work correctly with __resolveReference though and can cause unintentional errors.

When looking at the api reference of __resolveReference, context is the second argument when in GraphQLFieldResolver, context is the third argument.

e.g.

import { GraphQLFieldResolver } from 'graphql';

import * as Models from '../../db/models';
import * as Schema from '../generated/schema-types';
import { Context } from '../helpers/context';

/**
 * Given the id, resolves the Foo model.
 */
// NOTICE: This function is broken.
// GraphQLFieldResolver usage is wrong! Notice `context` should be second argument.
const __resolveReference: GraphQLFieldResolver<{ id: Schema.Foo['id'] }, Context, null> = async (
  reference,
  _args,
  context
): Promise<Models.Foo> => {
  return await context.fooLoader.load(reference.id);
};

• A simple, runnable reproduction! Please make a GitHub repository that anyone can clone and run and see the problem. Other great ways to demonstrate a reproduction are using our CodeSandbox template (https://codesandbox.io/s/apollo-server), or re-mixing our Glitch template (https://glitch.com/~apollo-launchpad). –>

[dncrews]

That’s because __resolveReference isn’t a GraphQLFieldResolver, it’s a GraphQLReferenceResolver (as defined in @apollo/federation), but I don’t know if that’s exported anywhere in a consumable way.

[DarkLite1]

Oh, my bad. I need to define an interface for each type:

interface UserInterface {
  id: string
  name: string
  username: string
  birthDate: string
}
// resolvers
User: {
    __resolveReference(object: UserInterface) {
      return users.find((user) => user.id === object.id)
    },
  },

I expected this to be done automatically. Anyway, sorry for hijacking the threat.