Add Day_4: 设计一个真正的Django小应用
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# Day_4 参考解答
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> 说明：这里是完整参考实现，按文件拆成四块。
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> 建议先自己写，卡住了再来看对应文件；看完后尝试不参考、重写一遍 views.py。
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---
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## 1. blog/models.py
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```python
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from django.db import models
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class Member(models.Model):
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"""社团成员"""
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nickname = models.CharField(max_length=50)
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email = models.EmailField(unique=True) # 邮箱唯一
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joined_at = models.DateTimeField(auto_now_add=True) # 创建时自动记录
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bio = models.TextField(blank=True, null=True) # 允许为空
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def __str__(self):
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return self.nickname
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class Article(models.Model):
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"""文章"""
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title = models.CharField(max_length=100)
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content = models.TextField()
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author = models.ForeignKey(
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Member,
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on_delete=models.CASCADE, # 作者被删 => 文章一起删
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related_name="articles", # member.articles.all() 能取到该作者全部文章
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)
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is_published = models.BooleanField(default=False)
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created_at = models.DateTimeField(auto_now_add=True)
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views = models.PositiveIntegerField(default=0) # 浏览量不会为负，用 PositiveIntegerField
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class Meta:
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ordering = ["-created_at"] # 新文章在前
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def __str__(self):
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return self.title
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class Comment(models.Model):
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"""评论"""
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content = models.TextField()
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article = models.ForeignKey(
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Article,
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on_delete=models.CASCADE,
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related_name="comments", # article.comments.all() 能取到该文章全部评论
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)
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commenter = models.ForeignKey(Member, on_delete=models.CASCADE)
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class Meta:
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ordering = ["id"] # 按评论先后排序
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def __str__(self):
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return f"评论({self.commenter})：{self.content[:20]}"
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```
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**思考题答案（供对照）**：
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1. `on_delete=models.CASCADE`：作者被删除时，他写的文章（以及文章下的评论）**级联删除**，
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不会留下指向不存在作者的孤儿数据。若改成 `SET_NULL`，删作者时文章的 `author`
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会被置为 `NULL`——但前提是外键字段允许为空，即
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`author = models.ForeignKey(Member, null=True, blank=True, on_delete=models.SET_NULL)`。
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2. `related_name="comments"`：定义**反向查询**的名字。有了它，从文章查评论就是
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`article.comments.all()`（不需要自己再写 `Comment.objects.filter(article=...)`）。
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---
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## 2. blog/views.py
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```python
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from django.db.models import Count, F
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from django.shortcuts import render, get_object_or_404, redirect
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from django.http import Http404
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from .models import Article, Comment, Member
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def index(request):
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# 只要已发布文章，按时间倒序（Meta.ordering 已生效，order_by 可省略，写上更明确）
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articles = Article.objects.filter(is_published=True).order_by("-created_at")
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return render(request, "blog/index.html", {"articles": articles})
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def article_detail(request, pk):
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# get_object_or_404 = get() + 捕获 DoesNotExist 后抛 Http404
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article = get_object_or_404(Article, pk=pk)
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# 浏览量 +1。为什么不用 article.views += 1; article.save()？
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# 两个用户同时点进来：两人都读到 views=10，各自 +1 后都写回 11，
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# 丢了 1 次访问——这就是"并发丢失更新"。
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# 用 F("views") + 1 是在数据库层完成"读旧值+1再写回"，
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# 原子操作，不会丢更新。
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Article.objects.filter(pk=pk).update(views=F("views") + 1)
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return render(request, "blog/detail.html", {"article": article})
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def add_comment(request, pk):
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article = get_object_or_404(Article, pk=pk)
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if request.method == "POST":
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content = request.POST.get("content", "").strip()
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if content:
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member = Member.objects.first() # 简化：还没做登录，先随便取一个成员
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Comment.objects.create(
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content=content,
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article=article,
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commenter=member,
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)
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return redirect("article_detail", pk=pk) # POST 后重定向，避免刷新重复提交
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def author_articles(request, nickname):
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author = get_object_or_404(Member, nickname=nickname)
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# 进阶：annotate 在数据库层完成"按文章分组数评论数"，
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# 返回的每篇文章都带一个动态属性 comment_count。
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articles = (
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Article.objects.filter(author=author)
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.annotate(comment_count=Count("comments")) # 反向查询名直接拿来用
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.order_by("-created_at")
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)
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return render(request, "blog/author.html", {"author": author, "articles": articles})
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```
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---
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## 3. 路由
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**项目级 mysite/urls.py**：
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```python
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from django.contrib import admin
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from django.urls import path, include
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urlpatterns = [
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path("admin/", admin.site.urls),
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path("", include("blog.urls")), # 把 / 开头的请求交给 blog 应用处理
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]
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```
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**应用级 blog/urls.py**：
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```python
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from django.urls import path
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from . import views
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app_name = "blog" # 配合 name 使用：模板里 {% url 'blog:article_detail' a.pk %}
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urlpatterns = [
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path("", views.index, name="index"),
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path("article/<int:pk>/", views.article_detail, name="article_detail"),
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path("article/<int:pk>/comment/", views.add_comment, name="add_comment"),
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path("author/<str:nickname>/", views.author_articles, name="author_articles"),
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]
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```
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**路由命名与反向解析**：`name` 给路由起了名字，模板和视图里用
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`{% url 'blog:article_detail' a.pk %}` / `redirect("article_detail", pk=pk)` 引用，
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而不是手写死 URL 字符串——这样以后改路径，代码不用跟着改。
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---
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## 4. 完整链路说明（题目三第 2 问示例答案）
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> 用户在浏览器输入 `/article/3/` 回车后：Django 首先在 **URL 路由表**中逐条匹配，
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> 命中 `path("article/<int:pk>/")`，把 `pk=3` 提取出来并调用对应的**视图函数**
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> `article_detail(request, pk=3)`。视图内执行 **ORM 查询**（`get_object_or_404` 对应
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> 一条 SELECT），拿到文章对象后调用 **`render`** 把数据填入模板、生成 HTML，
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> 最终打包成 **`HttpResponse`** 返回给浏览器渲染。
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---
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## 5. 进阶：index.html 模板片段
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```html
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{% if articles %}
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<ul>
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{% for a in articles %}
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<li>
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<a href="{% url 'blog:article_detail' a.pk %}">{{ a.title }}</a>
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<span>{{ a.created_at|date:"Y-m-d H:i" }}</span>
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· 作者：{{ a.author.nickname }}
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</li>
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{% endfor %}
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</ul>
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{% else %}
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<p>暂无文章，快来发布第一篇吧！</p>
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{% endif %}
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```
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---
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## 验收清单
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- [ ] `python manage.py makemigrations blog` 与 `migrate` 无报错
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- [ ] `runserver` 后，列表页 / 详情页 / 评论提交 / 作者页全部可访问
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- [ ] 删掉一个 Member 后，他的文章和评论也一起消失（验证 CASCADE）
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- [ ] 能口头讲清楚：两个用户同时点文章，浏览量不会"丢访问"
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