NestJS集成Mongoose启动失败：Cannot read 'tree'错误排查

问题描述 #

我正在学习NestJS，尝试使用Mongoose集成本地MongoDB，参考教程创建了coupon-service项目，启动时出现如下错误：

[Nest] 21089  - 05/05/2023, 12:25:36 AM     LOG [NestFactory] Starting Nest application...
[Nest] 21089  - 05/05/2023, 12:25:37 AM   ERROR [ExceptionHandler] Cannot read properties of undefined (reading 'tree')
TypeError: Cannot read properties of undefined (reading 'tree')
    at Model.Document.$__setSchema (/media/truongdd/ubuntu/learning/git/coupon-service/node_modules/mongoose/lib/document.js:3452:18)
    at Model.Document (/media/truongdd/ubuntu/learning/git/coupon-service/node_modules/mongoose/lib/document.js:88:10)
    at new Model (/media/truongdd/ubuntu/learning/git/coupon-service/node_modules/mongoose/lib/model.js:121:12)
    at Injector.instantiateClass (/media/truongdd/ubuntu/learning/git/coupon-service/node_modules/@nestjs/core/injector/injector.js:351:19)
    at callback (/media/truongdd/ubuntu/learning/git/coupon-service/node_modules/@nestjs/core/injector/injector.js:56:45)
    at Injector.resolveConstructorParams (/media/truongdd/ubuntu/learning/git/coupon-service/node_modules/@nestjs/core/injector/injector.js:136:24)
    at Injector.loadInstance (/media/truongdd/ubuntu/learning/git/coupon-service/node_modules/@nestjs/core/injector/injector.js:61:13)
    at Injector.loadProvider (/media/truongdd/ubuntu/learning/git/coupon-service/node_modules/@nestjs/core/injector/injector.js:88:9)
    at /media/truongdd/ubuntu/learning/git/coupon-service/node_modules/@nestjs/core/injector/instance-loader.js:56:13)
    at async Promise.all (index 4)

可能原因及解决方法 #

1. Schema定义或注册错误 #

• 问题：Schema类未用@Schema()装饰器正确修饰，字段定义不符合Mongoose规范，或者Module中未正确注册Schema，导致Model实例化时找不到合法的Schema对象。
• 解决：
  • 检查Schema文件，确保结构符合规范：

    import { Schema, Prop, SchemaFactory } from '@nestjs/mongoose';
    import { Document } from 'mongoose';
    
    @Schema()
    export class Coupon extends Document {
      @Prop({ required: true })
      code: string;
    
      @Prop({ required: true })
      discount: number;
    }
    
    export const CouponSchema = SchemaFactory.createForClass(Coupon);
    

  • 在对应Module中注册Schema：

    import { Module } from '@nestjs/common';
    import { MongooseModule } from '@nestjs/mongoose';
    import { Coupon, CouponSchema } from './schemas/coupon.schema';
    import { CouponService } from './coupon.service';
    
    @Module({
      imports: [MongooseModule.forFeature([{ name: Coupon.name, schema: CouponSchema }])],
      providers: [CouponService],
    })
    export class CouponModule {}
    

2. NestJS与Mongoose版本不兼容 #

• 问题：@nestjs/mongoose和mongoose版本不匹配，导致内部调用出现属性未定义的异常。
• 解决：
  • 查看package.json中的版本，参考NestJS官方文档确认兼容版本（比如@nestjs/mongoose@10.x对应mongoose@7.x+）。
  • 卸载现有依赖后重新安装兼容版本：

    npm uninstall @nestjs/mongoose mongoose
    npm install @nestjs/mongoose@适配版本 mongoose@对应兼容版本
    

3. Model注入方式错误 #

• 问题：在Service中直接注入Model类，未使用@InjectModel()装饰器，导致Nest无法正确实例化Mongoose Model。
• 解决：
  • 按规范在Service中注入Model：

    import { Injectable } from '@nestjs/common';
    import { InjectModel } from '@nestjs/mongoose';
    import { Model } from 'mongoose';
    import { Coupon } from './schemas/coupon.schema';
    
    @Injectable()
    export class CouponService {
      constructor(@InjectModel(Coupon.name) private couponModel: Model<Coupon>) {}
    }
    

4. Schema文件导出错误 #

• 问题：Schema文件未正确导出Schema对象或对应的类，导致Module注册时无法获取到有效Schema。
• 解决：检查Schema文件的导出语句，确保Coupon类和CouponSchema都通过export关键字正确导出，避免默认导出与命名导出混用导致的导入错误。

内容的提问来源于stack exchange，提问作者Đức Trường Đặng