节点-GraphQL-Ad.user字段类型必须为“输出类型"，但得到:未定义 [英] Node - GraphQL - Ad.user field type must be Output Type but got: undefined

问题描述

我似乎在AdType中使用UserType时遇到循环依赖问题.

I seem to be having circular dependency problems with using UserType inside AdType.

这是我的UserType文件:

This is my UserType file: UserType

这是我的AdType文件:

This is my AdType file: AdType

如果我尝试使用下面的代码，即使我正确导入了UserType，也会收到错误消息"Ad.user字段类型必须为Output Type，但得到:undefined".

If I try to use the code below I get the error "Ad.user field type must be Output Type but got: undefined" even tho I imported the UserType properly.

import { UserType } from '../User/user.graphql.model'
import { UserSchema } from '../User/user.mongoose.model'

const user =  {
  type: UserType,
  resolve(parentValue, args) {
    return UserSchema.findById(parentValue.user);
  }
};

//------------------------------
// Ad Type
//------------------------------
export const AdType = new GraphQLObjectType({
  name: 'Ad',
  fields: () => ({
    id,
    user,
    title,
    views,
    availability,
... more code

如果导入后我尝试在AdType内控制台记录UserType，它会显示为undefined，但是当我将其用作:

If I try to console log UserType inside AdType after importing it, it says undefined, but when I use it as:

//------------------------------
// Ad Type
//------------------------------
export const AdType = new GraphQLObjectType({
  name: 'Ad',
  fields: () => ({
    id,
    user: {
      type: UserType,
      resolve(parentValue, args) {
        return UserSchema.findById(parentValue.user);
      }
    },
    title,
... more code

它按预期工作，只是不允许我拉出代码来分隔常量. 我以相同的方式导入和使用的所有其他类型都可以按预期方式工作，将Ads导入到Users也能正常工作，但是将User导入到Ads中似乎很麻烦.两者基本上是相同的代码，只是信息不同.

it works as intended, it just doesn't allow me to pull out the code to separate constant. All other Types I import and use in the same manner work as expected, importing Ads into Users works as well, but importing User into Ads seems to break. It's basically the same code in both, just different info.

推荐答案

我已经在使用字段:()=>({})延迟加载字段，以避免出现循环依赖问题，所以这个问题真的困扰着我.

I am already using fields: () => ( { } ) to lazily load fields to avoid problems with circular dependencies, so this problem is really banging against my head.

但是您没有正确执行. Javascript没有惰性评估.这意味着user的值不是在调用函数时确定的，而是在评估const变量定义的时间点确定的.此时，变量UserType没有任何值，因此未定义.您的对象定义需要在调用函数时发生.如果仍然不清楚，我可以提供详细说明如何解析您的类型.

But you are not doing it correctly. Javascript does not have lazy evaluation. This means that the value of user is not determined when the function is called but at the point in time when the const variable definition is evaluated. At this point the variable UserType holds no value and thus is undefined. Your object definition needs to happen when the function is called. If it is still not clear I can offer to go into detail how your types are resolved.

尝试内联定义用户类型或使其成为函数:

Try defining the user type inline or make it a function:

const user = () => ({ type: UserType, /* ... */ })

export const AdType = new GraphQLObjectType({
  name: 'Ad',
  fields: () => ({
    id,
    user: user(),
    title,
    views,
    availability,

我不确定为什么将字段放入单独的常量中，您的代码看起来并不大，以至于提高了可读性，但是我当然是错的.

I am not sure why you pull your fields into seperate constants, your code does not seem that big that it improves readability but of course I can be wrong.

好的，让我们看看如何解决模块.为了使之更容易，我使用CJS，因为您很可能无论如何都要向下转换代码，而ES模块正慢慢进入节点.

Okay, let's see how modules are resolved. To make this easier I use CJS because you most likely transpile the code down anyways and ES modules are just slowly coming to node.

// user.graphql.model.js
const adModule = require('ad.graphql.model.js');

// Node tries to resolve ad.graphql.model.js
const userModule = require('user.graphql.model.js');
// Ups, this one has been resolved already and required this as dependency.
// We have no other choice than to assign an empty object here
// userModule is {}
const user =  {
  type: userModule.UserType, // undefined
  resolve(parentValue, args) {
    return UserSchema.findById(parentValue.user);
  }
};
// finish this module and return to user.graphql.model.js
// adModule now contains the resolved module
const adModule = require('ad.graphql.model.js');
// finish module and replace {} with actual module content in ad.graphql.model.js
// userModule contains UserType
const userModule = require('user.graphql.model.js');

const ads = {
  type: new GraphQLList(asModule.AdType), // GraphQLObjectType
}

// Now your Schema does build/inits itself after the config has been specified
// (and all modules have been resolved)
// imagine user being a function now taht is called on Schema init
const user = () => ({
  type: userModule.UserType, // GraphQLObjectType
  /* ... */
})

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