graphql-playground/server/schema/schema.js

const graphql = require('graphql');
const _ = require('lodash');

const {
  GraphQLObjectType,
  GraphQLString,
  GraphQLID,
  GraphQLInt,
  GraphQLList,
  GraphQLSchema
} = graphql;

// temp data
const books = [
  { name: 'Name of the Wind', genre: 'Fantasy', id: '1', authorId: '1' },
  { name: 'The Final Empire', genre: 'Fantasy', id: '2', authorId: '2' },
  { name: 'The Hero of Ages', genre: 'Fantasy', id: '4', authorId: '2' },
  { name: 'The Long Earth', genre: 'Sci-Fi', id: '3', authorId: '3' },
  { name: 'The Colour of Magic', genre: 'Fantasy', id: '5', authorId: '3' },
  { name: 'The Light Fantastic', genre: 'Fantasy', id: '6', authorId: '3' }
];
const authors = [
  { name: 'Patrick Rothfuss', age: 44, id: '1' },
  { name: 'Brandon Sanderson', age: 42, id: '2' },
  { name: 'Terry Pratchett', age: 66, id: '3' }
];

const BookType = new GraphQLObjectType({
  name: 'Book',
  fields: () => ({
    id: { type: GraphQLID },
    name: { type: GraphQLString },
    genre: { type: GraphQLString },
    author: {
      type: AuthorType,
      resolve (parent, args) {
        return _.find(authors, { id: parent.authorId });
      }
    }
  })
});

const AuthorType = new GraphQLObjectType({
  name: 'Author',
  fields: () => ({
    id: { type: GraphQLID },
    name: { type: GraphQLString },
    age: { type: GraphQLInt },
    books: {
      type: new GraphQLList(BookType),
      resolve (parent, args) {
        return _.filter(books, { authorId: parent.id });
      }
    }
  })
});

const RootQuery = new GraphQLObjectType({
  name: 'RootQueryType',
  fields: {
    book: {
      type: BookType,
      args: { id: { type: GraphQLID } },
      resolve (parent, args) {
        return _.find(books, { id: args.id });
      }
    },
    author: {
      type: AuthorType,
      args: { id: { type: GraphQLID } },
      resolve (parent, args) {
        return _.find(authors, { id: args.id });
      }
    },
    books: {
      type: new GraphQLList(BookType),
      resolve (parent, args) {
        return books;
      }
    },
    authors: {
      type: new GraphQLList(AuthorType),
      resolve (parent, args) {
        return authors;
      }
    }
  }
});

module.exports = new GraphQLSchema({
  query: RootQuery
});