djnago orm 一对多关系使用

在 Django 中，一对多关系通常通过外键（ForeignKey）来实现。以下是一个完整的示例，展示如何定义模型、创建一对多关系，并使用 Django ORM 进行连表查询。

1. 定义模型

假设我们有两个模型：Author 和 Book。一个作者可以有多本书，这就是一对多的关系。

models.py

from django.db import models

class Author(models.Model):
    name = models.CharField(max_length=100)
    email = models.EmailField()

    def __str__(self):
        return self.name

class Book(models.Model):
    title = models.CharField(max_length=200)
    publication_date = models.DateField()
    author = models.ForeignKey(Author, related_name='books', on_delete=models.CASCADE)

    def __str__(self):
        return self.title

2. 创建和应用迁移

在定义好模型后，需要创建和应用数据库迁移。

python manage.py makemigrations
python manage.py migrate

3. 创建一些示例数据

你可以使用 Django 管理后台或 Django shell 来创建一些示例数据。

使用 Django shell

python manage.py shell

from myapp.models import Author, Book

# 创建作者
author1 = Author.objects.create(name="Author One", email="author1@example.com")
author2 = Author.objects.create(name="Author Two", email="author2@example.com")

# 创建书籍
Book.objects.create(title="Book One", publication_date="2023-01-01", author=author1)
Book.objects.create(title="Book Two", publication_date="2023-02-01", author=author1)
Book.objects.create(title="Book Three", publication_date="2023-03-01", author=author2)

4. 连表查询

现在可以使用 Django ORM 进行连表查询。

views.py

from django.shortcuts import render
from myapp.models import Author, Book

def author_books(request, author_id):
    # 获取指定作者
    author = Author.objects.get(id=author_id)
    
    # 获取该作者的所有书籍
    books = author.books.all()
    
    return render(request, 'author_books.html', {'author': author, 'books': books})

urls.py

from django.urls import path
from myapp import views

urlpatterns = [
    path('author/<int:author_id>/', views.author_books, name='author_books'),
]

author_books.html

<!DOCTYPE html>
<html>
<head>
    <title>Author Books</title>
</head>
<body>
    <h1>Books by {{ author.name }}</h1>
    <ul>
        {% for book in books %}
            <li>{{ book.title }} ({{ book.publication_date }})</li>
        {% endfor %}
    </ul>
</body>
</html>

5. 复杂查询示例

如果需要进行更复杂的查询，比如获取所有书籍及其对应的作者，可以使用 select_related 或 prefetch_related。

使用 select_related

def all_books(request):
    # 获取所有书籍及其对应的作者
    books = Book.objects.select_related('author').all()
    
    return render(request, 'all_books.html', {'books': books})

all_books.html

<!DOCTYPE html>
<html>
<head>
    <title>All Books</title>
</head>
<body>
    <h1>All Books</h1>
    <ul>
        {% for book in books %}
            <li>{{ book.title }} by {{ book.author.name }} ({{ book.publication_date }})</li>
        {% endfor %}
    </ul>
</body>
</html>

6. 更新 urls.py

urlpatterns = [
    path('author/<int:author_id>/', views.author_books, name='author_books'),
    path('books/', views.all_books, name='all_books'),
]