mergeSchema.js

import { mergeSchemas, delegateToSchema } from 'graphql-tools';

const extensionSchema = `
  interface Node {
    id: ID!
  }

  extend type User implements Node {
    id: ID!
  }

  extend type Blog implements Node {
    id: ID!
  }

  extend type Query {
    node(id: ID!): Node
  }
`;

const schema = mergeSchemas({
  schemas: [serviceSchema, extensionSchema],
  resolvers: {
    Query: {
      node(parent, { id }, context, info) {
        const { type, localId } = deserializeGlobalId(id);
        let fieldName;
        if (type === 'Blog') {
          fieldName = 'blogById',
        } else if (type === 'User') {
          fieldName = 'userById',
        } else {
          throw new Error('Invalid global id');
        }
        return delegateToSchema({
          schema: serviceSchema,
          operation: 'query',
          fieldName,
          args: { id: localId },
          context,
          info,       
         });
      },
    },
    // ... more resolvers
 },
});