import { transformSchemaFederation } from 'graphql-transform-federation';
import { delegateToSchema } from 'graphql-tools';

const schemaWithoutFederation = // your existing executable schema

const federationSchema = transformSchemaFederation(schemaWithoutFederation, {
  Query: {
    // Ensure the root queries of this schema show up the combined schema
    extend: true,
  },
  Product: {
    // extend Product {
    extend: true,
    // Product @key(fields: "id") {
    keyFields: ['id'],
    fields: {
      // id: Int! @external
      id: {
        external: true
      }
    },
    resolveReference({ id }, context, info) {
      return delegateToSchema({
        schema: info.schema,
        operation: 'query',
        fieldName: 'productById',
        args: {
          id,
        },
        context,
        info,
      });
    },
  },
});

const federationSchema = transformSchemaFederation(schemaWithoutFederation, {
  Product: {
    // Product @key(fields: "id") {
    keyFields: ['id'],
  },
});

import { GraphQLSchema } from 'graphql';
import { GraphQLReferenceResolver } from '@apollo/federation/dist/types';

interface FederationFieldConfig {
  external?: boolean;
  provides?: string;
  requires?: string;
}

interface FederationFieldsConfig {
  [fieldName: string]: FederationFieldConfig;
}

interface FederationObjectConfig<TContext> {
  // An array so you can add multiple @key(...) directives
  keyFields?: string[];
  extend?: boolean;
  resolveReference?: GraphQLReferenceResolver<TContext>;
  fields?: FederationFieldsConfig;
}

interface FederationConfig<TContext> {
  [objectName: string]: FederationObjectConfig<TContext>;
}

function transformSchemaFederation<TContext>(
  schema: GraphQLSchema,
  federationConfig: FederationConfig<TContext>,
): GraphQLSchema;