“预期未定义为 GraphQL 类型”的含义？

Posted 2023-03-08

【中文标题】“预期未定义为 GraphQL 类型”的含义？

【英文标题】：Meaning of "Expected undefined to be a GraphQL type"?

【发布时间】：2021-03-17 17:03:14

【问题描述】：

我正在使用 graphql 和 graphql-express 设置我的第一个 GraphQL 端点。我的 MongoDB 架构有两种类型：User 和 Card，其中一个 User 可以拥有许多 Card。我设置了根类型和解析器，没有任何问题。我可以查询和改变数据库中的用户和卡片。

当我尝试在用户中嵌套卡片时出现问题，我得到Error: Expected undefined to be a GraphQL type。该错误仅在我将UserType.fields.cards.type 设置为graphql.GraphQLList 的实例时出现，但我在QueryType.js 中有一个相同的field 值，并且工作正常。

查看 Stack Overflow 上的其他问题，看来我的问题可能是我的 User 和 Card 类型之间的循环引用，但我正在努力寻找任何问题在我的代码中。我这里有什么遗漏吗？

完整的堆栈跟踪是：

Error: Expected undefined to be a GraphQL type.
    at assertType (/Users/QuestionMark/Documents/flash-cards/node_modules/graphql/type/definition.js:97:11)
    at new GraphQLList (/Users/QuestionMark/Documents/flash-cards/node_modules/graphql/type/definition.js:323:19)
    at GraphQLList (/Users/QuestionMark/Documents/flash-cards/node_modules/graphql/type/definition.js:325:12)
    at fields (/Users/QuestionMark/Documents/flash-cards/server/schema/types/UserType.js:16:13)
    at resolveThunk (/Users/QuestionMark/Documents/flash-cards/node_modules/graphql/type/definition.js:480:40)
    at defineFieldMap (/Users/QuestionMark/Documents/flash-cards/node_modules/graphql/type/definition.js:692:18)
    at GraphQLObjectType.getFields (/Users/QuestionMark/Documents/flash-cards/node_modules/graphql/type/definition.js:633:27)
    at collectReferencedTypes (/Users/QuestionMark/Documents/flash-cards/node_modules/graphql/type/schema.js:366:81)
    at collectReferencedTypes (/Users/QuestionMark/Documents/flash-cards/node_modules/graphql/type/schema.js:368:9)
    at new GraphQLSchema (/Users/QuestionMark/Documents/flash-cards/node_modules/graphql/type/schema.js:153:7)
    at Object.<anonymous> (/Users/QuestionMark/Documents/flash-cards/server/schema/schema.js:5:16)
    at Module._compile (internal/modules/cjs/loader.js:689:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:700:10)
    at Module.load (internal/modules/cjs/loader.js:599:32)
    at tryModuleLoad (internal/modules/cjs/loader.js:538:12)
    at Function.Module._load (internal/modules/cjs/loader.js:530:3)

这是我的相关代码供参考：

// ./UserType.js
const 
  GraphQLObjectType, GraphQLList, GraphQLID, GraphQLString
 = require('graphql');
const  CardType  = require('./CardType');
const  cardsResolver  = require('../resolvers/userResolvers');

const UserType = new GraphQLObjectType(
  name: 'User',
  fields: () => (
    _id:  type: GraphQLID ,
    username:  type: GraphQLString ,
    password:  type: GraphQLString ,
    firstname:  type: GraphQLString ,
    lastname:  type: GraphQLString ,
    cards: 
      type: GraphQLList(CardType),
      resolve: cardsResolver
    
  )
);

module.exports = UserType;

// ./CardType.js
const  GraphQLObjectType, GraphQLID, GraphQLString  = require('graphql');

const CardType = new GraphQLObjectType(
  name: 'Card',
  fields: 
    _id:  type: GraphQLID ,
    title:  type: GraphQLString ,
    body:  type: GraphQLString ,
    group:  type: GraphQLString 
  
);

module.exports = CardType;

// ./root/QueryType
const 
  GraphQLObjectType,
  GraphQLList,
  GraphQLID,
  GraphQLString
 = require('graphql');
const UserType = require('../UserType');
const CardType = require('../CardType');
const 
  userResolver,
  usersResolver,
  cardResolver,
  cardsResolver
 = require('../../resolvers/root/queryResolvers');

const QueryType = new GraphQLObjectType(
  name: 'Query',
  fields: 
    user: 
      type: UserType,
      args: 
        _id:  type: GraphQLID ,
        username:  type: GraphQLString 
      ,
      resolve: userResolver
    ,
    users: 
      type: GraphQLList(UserType),
      resolve: usersResolver
    ,
    card: 
      type: CardType,
      args: 
        _id:  type: GraphQLID 
      ,
      resolve: cardResolver
    ,
    cards: 
      type: GraphQLList(CardType),
      resolve: cardsResolver
    
  
);

module.exports = QueryType;

【参考方案1】：

在 UserType.js 中，我解构了 require('./CardType') 的输出，导致 CardType 未定义。 GraphQLList 期望 GraphQLObjectType 的实例，但收到 undefined，因此出现错误。

    const  CardType  = require('./CardType'); // old code
    const CardType = require('./CardType'); // new code