Django:如何从model表单上的选择中获取外键ID？

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)
    author = models.ForeignKey(Author, on_delete=models.CASCADE)

from django.shortcuts import render
from .models import Book

def create_book(request):
    if request.method == 'POST':
        title = request.POST['title']
        author_id = request.POST['author']  # 获取用户选择的外键ID
        book = Book(title=title, author_id=author_id)
        book.save()
        # 其他处理逻辑...
    else:
        # 渲染表单页面...
        return render(request, 'create_book.html')

<select name="author">
  {% for author in authors %}
    <option value="{{ author.id }}" {% if book.author_id == author.id %}selected{% endif %}>{{ author.name }}</option>
  {% endfor %}
</select>