使用graphql-compose-mongoose报错：期望[object Object]为GraphQL Schema

我是graphql-compose新手，尝试基于简单的Mongoose Schema搭建首个服务：

GraphQL 代码 #

import mongoose from 'mongoose' 
import { composeWithMongoose} from 'graphql-compose-mongoose' 
import { schemaComposer } from 'graphql-compose' 
const db = require( '../models/db' ) 
//const mongoose = require('mongoose'); 
const folderDAO = mongoose.model('folder'); 
const customizationOptions = {}; // left it empty for simplicity, described below 
const folderTC = composeWithMongoose(folderDAO, customizationOptions); 
schemaComposer.rootQuery().addFields({ 
  folderOne: folderTC.getResolver('findOne'), 
}) 
const graphqlSchema = schemaComposer.buildSchema() 
console.log("Schema built : ", graphqlSchema ) 
export default graphqlSchema 

服务器代码 #

const express = require('express'); 
const graphqlHTTP = require('express-graphql') 
const GraphQLSchema = require('./app_api/routes/graphql') 
app.use('/graphql', graphqlHTTP({ 
  schema: GraphQLSchema, 
  graphiql: true, 
  formatError: error => ({ 
    message: error.message, 
    locations: error.locations, 
    stack: error.stack ? error.stack.split('\n') : [], 
    path: error.path 
  }) 
})); 

在GraphiQL中执行的查询 #

{ 
  folderOne(filter: {}, sort: _ID_ASC) { 
    name 
  } 
} 

出现的错误信息 #

{ 
  "errors": [ 
    { 
      "message": "Expected [object Object] to be a GraphQL schema.", 
      "stack": [ 
        "Error: Expected [object Object] to be a GraphQL schema.", 
        " at invariant (/Users/zied/work/share_place/node_modules/graphql/jsutils/invariant.js:19:11)", 
        " at validateSchema (/Users/zied/work/share_place/node_modules/graphql/type/validate.js:55:60)", 
        " at assertValidSchema (/Users/zied/work/share_place/node_modules/graphql/type/validate.js:80:16)", 
        " at validate (/Users/zied/work/share_place/node_modules/graphql/validation/validate.js:58:35)", 
        " at /Users/zied/work/share_place/node_modules/express-graphql/dist/index.js:139:52", 
        " at <anonymous>", 
        " at process._tickDomainCallback (internal/process/next_tick.js:228:7)" 
      ] 
    } 
  ] 
} 

请问我遗漏了什么？

这个问题的核心是模块导入导出的格式不匹配，导致express-graphql拿到的不是一个有效的GraphQL Schema对象。

你看，你的graphql.js用的是ES Module的export default导出Schema，但服务器代码里用的是CommonJS的require去导入。在Node.js环境中，用require导入ES Module的默认导出时，需要通过.default属性来获取实际的Schema对象。

具体修复步骤： #

修改服务器代码里的导入语句：

// 原来的写法
const GraphQLSchema = require('./app_api/routes/graphql')

// 修改为
const GraphQLSchema = require('./app_api/routes/graphql').default

另外还有一种可选的调整方式：如果你的项目想统一用ES Module，可以在package.json里添加"type": "module"，然后把服务器代码改成ES Module的语法：

import express from 'express';
import { graphqlHTTP } from 'express-graphql';
import GraphQLSchema from './app_api/routes/graphql';

两种方式选一种即可，核心就是保证导入导出的模块系统一致，让express-graphql能拿到真正的GraphQL Schema实例，而不是一个包含默认导出的包装对象。

内容的提问来源于stack exchange，提问作者Zied Hamdi