Django prefetch

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

    
    Title


{% 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 %}

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/