Service 层

这一课你将能做到

1. 把题库 CRUD 与筛选、作答判分等业务规则全部放进 @Injectable Service，Controller 保持薄
2. 在缺失资源、参数越界、数据冲突时抛语义化异常（404 / 400 / 409）
3. 用 Test.createTestingModule 不启动 HTTP 就能验证 Service 行为

为什么要学

Service 放「业务规则与编排」：404、下标越界、重复题干、判分、正确率。Controller 只翻译 HTTP，Repository 只碰持久化。

分层最大回报是可测试性：业务不依赖请求对象和数据库连接，单测几秒跑完，与 Spring @Service 同一思路。

先建立图像

Controller   取参、状态码、转调
Service      规则、编排、未来的事务边界
Repository   持久化（数据库课换 ORM）

应该放在 Service	不应该放在 Service
业务规则（下标范围、查重）	req / res 等 HTTP 对象
抛语义异常	路由与状态码拼装
调用其他 Service	JWT 解析（Guard 的职责）

跟着做

步骤 1：实现完整的 QuestionsService

替换 src/questions/questions.service.ts：

import {
  BadRequestException,
  ConflictException,
  Injectable,
  NotFoundException,
} from "@nestjs/common";
import { CreateQuestionDto } from "./dto/create-question.dto";
import { UpdateQuestionDto } from "./dto/update-question.dto";

export type Difficulty = "easy" | "medium" | "hard";

export interface Question {
  id: number; stem: string; options: string[];
  answerIndex: number; difficulty: Difficulty; tags: string[];
}

export interface SearchQuery {
  difficulty?: Difficulty; tag?: string; page: number;
}

@Injectable()
export class QuestionsService {
  private items: Question[] = [{
    id: 1,
    stem: "TypeScript 中，用来描述对象形状的类型是？",
    options: ["interface", "class", "enum", "namespace"],
    answerIndex: 0, difficulty: "easy", tags: ["typescript"],
  }];
  private nextId = 2;

  search({ difficulty, tag, page }: SearchQuery) {
    const size = 10;
    const data = this.items.filter(
      (q) =>
        (!difficulty || q.difficulty === difficulty) &&
        (!tag || q.tags.includes(tag)),
    );
    const start = (page - 1) * size;
    return { total: data.length, page, data: data.slice(start, start + size) };
  }

  findAll(): Question[] {
    return this.items;
  }

  findOne(id: number): Question {
    const question = this.items.find((q) => q.id === id);
    if (!question) {
      throw new NotFoundException(`题目 ${id} 不存在`);
    }
    return question;
  }

  create(dto: CreateQuestionDto): Question {
    this.assertAnswerIndex(dto.options.length, dto.answerIndex);
    if (this.items.some((q) => q.stem === dto.stem)) {
      // 题干重复属于资源冲突，409 比 400 贴切
      throw new ConflictException("相同题干的题目已存在");
    }
    const question: Question = { id: this.nextId++, ...dto, tags: dto.tags ?? [] };
    this.items.push(question);
    return question;
  }

  update(id: number, dto: UpdateQuestionDto): Question {
    const question = this.findOne(id);
    const options = dto.options ?? question.options;
    const answerIndex = dto.answerIndex ?? question.answerIndex;
    this.assertAnswerIndex(options.length, answerIndex);
    // JSON 往返会自动剔除值为 undefined（PATCH 未传）的字段
    Object.assign(question, JSON.parse(JSON.stringify(dto)), { options, answerIndex });
    return question;
  }

  remove(id: number): void {
    this.findOne(id); // 不存在直接抛 404
    this.items = this.items.filter((q) => q.id !== id);
  }

  private assertAnswerIndex(optionCount: number, answerIndex: number) {
    if (answerIndex < 0 || answerIndex >= optionCount) {
      throw new BadRequestException(
        `answerIndex 必须在 0 到 ${optionCount - 1} 之间`,
      );
    }
  }
}