如何在django管理中对只读M2M字段进行排序

from django.db import models

class Book(models.Model):
    title = models.CharField(max_length=100)

class Author(models.Model):
    name = models.CharField(max_length=100)
    books = models.ManyToManyField(Book, related_name='authors')

from django.contrib import admin
from .models import Author

class AuthorAdmin(admin.ModelAdmin):
    readonly_fields = ('books',)

    def books(self, obj):
        return ", ".join([book.title for book in obj.books.all()])

    books.short_description = 'Books'

admin.site.register(Author, AuthorAdmin)