NestJS中@Prop设为ObjectId时API启动失败，设为string正常

问题现象 #

在NestJS中使用Mongoose定义Schema时，将@Prop的字段类型设置为直接从mongoose导入的ObjectId时，API无法正常启动；将类型改为mongoose.Types.ObjectId后，API恢复正常加载。

错误代码示例 #

import { Prop, Schema, SchemaFactory } from "@nestjs/mongoose";
import { Document, ObjectId } from "mongoose";

export type FriendRequestDocument = FriendRequest & Document;

@Schema({collection: "friendRequests"})
export class FriendRequest {
  @Prop()
  author: ObjectId;

  @Prop()
  friend_id: ObjectId;

  @Prop()
  request_at: Date;
}

export const FriendRequestSchema = SchemaFactory.createForClass(FriendRequest);

修复后的代码 #

import { Prop, Schema, SchemaFactory } from "@nestjs/mongoose";
import mongoose, { Document, ObjectId } from "mongoose";

export type FriendRequestDocument = FriendRequest & Document;

@Schema({collection: "friendRequests"})
export class FriendRequest {
  @Prop()
  author: mongoose.Types.ObjectId;

  @Prop()
  friend_id: mongoose.Types.ObjectId;

  @Prop()
  request_at: Date;
}

export const FriendRequestSchema = SchemaFactory.createForClass(FriendRequest);

原因解析 #

1. 类型与运行时构造函数的区别：从mongoose直接导入的ObjectId是TypeScript的类型别名，仅用于编译时的类型检查，在运行时会被完全擦除，不存在实际的对象或构造函数。
2. SchemaFactory的运行时依赖：NestJS的SchemaFactory.createForClass方法在解析Schema类时，需要读取字段的运行时类型信息来生成对应的MongoDB Schema规则。如果字段类型是仅编译时存在的类型别名，SchemaFactory无法识别该类型，进而抛出类型解析错误，导致API启动失败。
3. mongoose.Types.ObjectId的作用：mongoose.Types.ObjectId是Mongoose提供的运行时构造函数，SchemaFactory可以正确识别它，并将字段映射为MongoDB的ObjectId类型，因此能正常生成Schema并启动API。

可选优化方案 #

如果希望在类型声明时使用更简洁的ObjectId类型别名，同时保证运行时解析正常，可以显式在@Prop装饰器中指定类型：

import { Prop, Schema, SchemaFactory } from "@nestjs/mongoose";
import mongoose, { Document, ObjectId } from "mongoose";

export type FriendRequestDocument = FriendRequest & Document;

@Schema({collection: "friendRequests"})
export class FriendRequest {
  @Prop({ type: mongoose.Types.ObjectId })
  author: ObjectId;

  @Prop({ type: mongoose.Types.ObjectId })
  friend_id: ObjectId;

  @Prop()
  request_at: Date;
}

export const FriendRequestSchema = SchemaFactory.createForClass(FriendRequest);

内容的提问来源于stack exchange，提问作者Alms