如何在GraphQL中跨rest数据源维护请求上下文

const { ApolloServer, gql } = require('apollo-server');
const { RESTDataSource } = require('apollo-datasource-rest');
const DataLoader = require('dataloader');

// 定义GraphQL类型和查询
const typeDefs = gql`
  type User {
    id: ID!
    name: String!
    posts: [Post!]!
  }

  type Post {
    id: ID!
    title: String!
    body: String!
  }

  type Query {
    user(id: ID!): User
  }
`;

// 定义数据加载器类
class UserAPI extends RESTDataSource {
  constructor() {
    super();
    this.baseURL = 'https://api.example.com/users/';
  }

  // 加载用户数据
  async getUser(id) {
    return this.get(id);
  }
}

class PostAPI extends RESTDataSource {
  constructor() {
    super();
    this.baseURL = 'https://api.example.com/posts/';
  }

  // 加载用户的帖子数据
  async getPostsByUserIds(userIds) {
    return Promise.all(userIds.map(userId => this.get(`?userId=${userId}`)));
  }
}

// 创建数据加载器实例
const userLoader = new DataLoader(ids => userAPI.getUsers(ids));
const postLoader = new DataLoader(ids => postAPI.getPostsByUserIds(ids));

// 定义解析器
const resolvers = {
  Query: {
    user: async (_, { id }, { dataSources }) => {
      // 使用数据加载器加载用户数据
      const user = await dataSources.userAPI.getUser(id);
      return user;
    },
  },
  User: {
    posts: async (user, _, { dataSources }) => {
      // 使用数据加载器加载用户的帖子数据
      const posts = await dataSources.postAPI.getPostsByUserIds([user.id]);
      return posts;
    },
  },
};

// 创建Apollo服务器
const server = new ApolloServer({
  typeDefs,
  resolvers,
  dataSources: () => ({
    userAPI: new UserAPI(),
    postAPI: new PostAPI(),
  }),
  context: () => ({
    dataSources: {
      userAPI: userLoader,
      postAPI: postLoader,
    },
  }),
});

// 启动服务器
server.listen().then(({ url }) => {
  console.log(`Server ready at ${url}`);
});