prefetch_related的作用

prefetch_related()是 Django ORM 中用于优化查询性能的另一个重要方法，尤其在处理多对多（ManyToMany）关系和反向关系时非常有用。它允许你预加载相关对象，从而减少数据库查询次数。

1，创建应用

Test/app13

 python manage.py startapp app13

2，注册应用

Test/Test/settings.py

3，添加应用路由

from django.contrib import admin from django.urls import path, include  urlpatterns = [      path('app13/', include('app13.urls')), ] 

4，添加模型

Test/app13/models.py

from django.db import models  class Author(models.Model):     name = models.CharField(max_length=100)  class Book(models.Model):     title = models.CharField(max_length=100)     authors = models.ManyToManyField(Author)  class Review(models.Model):     book = models.ForeignKey(Book, on_delete=models.CASCADE)     rating = models.IntegerField()

5，添加视图函数

Test/app13/views.py

from django.shortcuts import render from .models import Book, Author, Review  def book_list(request):     # 使用 prefetch_related 预加载作者和评论信息     books = Book.objects.prefetch_related('authors', 'review_set').all()      # 准备传递给模板的上下文     context = {         'books': books,     }      # 渲染并返回响应     return render(request, '13/book_list.html', context)

 6，添加html代码

Test/templates/13/book_list.html

                {% for book in books %}     
{{ book.title }}
     
Authors:
     
     {% for author in book.authors.all %}         
{{ author.name }}
     {% endfor %}     
     
Reviews:
     
     {% for review in book.review_set.all %}         
Rating: {{ review.rating }}
     {% endfor %}     
 {% endfor %}    

7，添加路由地址

 Test/app13/urls.py

from django.urls import path from . import views  urlpatterns = [      path('book_list/', views.book_list, name='book_list'), ]

 8，添加数据

Test/populate_db.py

import random   import os import django  os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'Test.settings') django.setup() from app13.models import Author, Book, Review    # 创建随机作者 for _ in range(10):     author = Author(name=f'Author {_}')     author.save()  # 创建随机书籍 for _ in range(20):     book = Book(title=f'Book Title {_}')     book.save()     # 随机选择1-3个作者     authors = Author.objects.all()     book.authors.set(random.sample(list(authors), random.randint(1, 3)))  # 创建随机评论 for _ in range(50):     review = Review(         book=random.choice(Book.objects.all()),         rating=random.randint(1, 5)     )     review.save()  

 9，访问页面

http://127.0.0.1:8000/app13/book_list/