Snippet from the Advanced GraphQL workshop I run at OK GROW! - https://www.okgrow.com/graphql

schema-stitching.js

// define a way to connect to the external GraphQL API
const link = new HttpLink({
  uri: 'https://external-api.com/graphql',
  fetch,
});

// create a remote schema from the external GraphQL API
const remoteSchema = makeRemoteExecutableSchema({
  schema: await introspectSchema(link),
  link,
});

// create the local schema
const localSchema = makeExecutableSchema({ typeDefs, resolvers });

// from a local type, how to access a remote type
const linkSchema = `
  extend type Location {
    weather: Weather
  }
`;

// the schema that we'll serve
const schema = mergeSchemas({
  // all three above schemas together
  schemas: [remoteSchema, localSchema, linkSchema],
  resolvers: mergeInfo => ({
    // specific resolver for the type definition linking the local schema & the remote schema
    Location: {
      weather: {
        fragment:
          'fragment WeatherLocation on Location { longitude latitude }',
        resolve: ({ longitude, latitude }, args, context, info) => {
          return mergeInfo.delegate(
            'query',
            'weather',
            {
              coords: {
                longitude,
                latitude,
              },
            },
            context,
            info
          );
        },
      },
    },
  }),
});