综合练习

这一课你将能做到

1. 为「在线判题学习平台」设计一套 MongoDB 文档模型，包含用户、课时、提交记录、排行榜四个集合
2. 用索引优化三个高频查询，并用聚合管道输出一张「课时通过率 + 用户排名」双报表
3. 用 Change Streams 监听新提交，触发排行榜实时更新，形成闭环

为什么要学

前 11 课是零件，这一课是整车。学习平台的数据形状天然适合 MongoDB：提交记录是文档，排行榜要实时，用户进度要快速更新。把 11 课的知识点串成一个可运行的最小系统，才算真正学会。

先建立图像

数据模型（4 个集合）：

users          lessons            submissions            leaderboard
├─ _id         ├─ _id             ├─ _id                 ├─ _id (userId)
├─ nickname    ├─ slug            ├─ userId              ├─ nickname
├─ avatar      ├─ title           ├─ lessonSlug          ├─ totalScore
└─ createdAt   ├─ difficulty      ├─ status              ├─ passCount
               ├─ tags[]          ├─ score               ├─ lastSubmitAt
               └─ trackSlug       ├─ attempts            └─ updatedAt
                                  ├─ cases[]（内嵌）
                                  └─ submittedAt

数据流：
提交 → submissions (insert) → Change Stream → 累加 leaderboard (upsert + $inc)
                                            ↘ Kafka → 异步通知/风控
查询排行榜 → leaderboard 单集合（无 JOIN） → 按 totalScore 倒序取前 10

跟着做

步骤 1：创建集合与初始数据

use codeforge

// 课时字典
db.lessons.drop()
db.lessons.insertMany([
  { slug: "mdbrm-intro",    title: "MongoDB 概览",   difficulty: 1, trackSlug: "mongodb-roadmap-essentials" },
  { slug: "mdbrm-types",    title: "BSON 与类型",    difficulty: 2, trackSlug: "mongodb-roadmap-essentials" },
  { slug: "mdbrm-crud",     title: "CRUD 操作",      difficulty: 2, trackSlug: "mongodb-roadmap-essentials" },
  { slug: "mdbrm-query",    title: "查询运算符",     difficulty: 3, trackSlug: "mongodb-roadmap-essentials" },
  { slug: "mdbrm-agg",      title: "聚合管道",       difficulty: 3, trackSlug: "mongodb-roadmap-essentials" }
])

// 用户
db.users.drop()
db.users.insertMany([
  { _id: 1024, nickname: "小林", avatar: "a.png", createdAt: new Date("2026-09-01") },
  { _id: 2048, nickname: "阿明", avatar: "b.png", createdAt: new Date("2026-09-15") },
  { _id: 4096, nickname: "老王", avatar: "c.png", createdAt: new Date("2026-10-01") }
])

// 提交记录（带内嵌 cases）
db.submissions.drop()
db.submissions.insertMany([
  { userId: 1024, lessonSlug: "mdbrm-intro", status: "accepted", score: 100, attempts: 1,
    cases: [{name:"c1",passed:true,costMs:12},{name:"c2",passed:true,costMs:8}],
    submittedAt: new Date("2026-10-01T08:00:00Z") },
  { userId: 1024, lessonSlug: "mdbrm-types", status: "failed",   score: 40,  attempts: 2,
    cases: [{name:"c1",passed:true,costMs:15},{name:"c2",passed:false,costMs:300}],
    submittedAt: new Date("2026-10-02T09:00:00Z") },
  { userId: 1024, lessonSlug: "mdbrm-types", status: "accepted", score: 88,  attempts: 1,
    cases: [{name:"c1",passed:true,costMs:10},{name:"c2",passed:true,costMs:25}],
    submittedAt: new Date("2026-10-03T10:00:00Z") },
  { userId: 2048, lessonSlug: "mdbrm-intro", status: "accepted", score: 95,  attempts: 1,
    cases: [{name:"c1",passed:true,costMs:11},{name:"c2",passed:true,costMs:9}],
    submittedAt: new Date("2026-10-04T11:00:00Z") },
  { userId: 2048, lessonSlug: "mdbrm-crud",  status: "accepted", score: 78,  attempts: 2,
    cases: [{name:"c1",passed:true,costMs:20},{name:"c2",passed:false,costMs:180}],
    submittedAt: new Date("2026-10-05T14:00:00Z") },
  { userId: 4096, lessonSlug: "mdbrm-intro", status: "failed",   score: 30,  attempts: 3,
    cases: [{name:"c1",passed:false,costMs:200},{name:"c2",passed:false,costMs:400}],
    submittedAt: new Date("2026-10-06T16:00:00Z") }
])

// 排行榜（初始为空，由 Change Stream 或聚合初始化）
db.leaderboard.drop()