Django多对多字段保存前校验报错：需先获取Classroom的id值

Django的多对多关系依赖主模型实例的主键（id）维护中间表关联，新建Classroom实例时，未保存到数据库前没有主键，直接访问self.faculty或self.students的关联集合会触发ValueError。

通过自定义ModelForm在表单层面做校验，避免直接操作未保存实例的多对多字段：

1. 定义Classroom的ModelForm，添加字段校验逻辑

from django import forms
from .models import Classroom

class ClassroomForm(forms.ModelForm):
    class Meta:
        model = Classroom
        fields = "__all__"

    def clean_faculty(self):
        faculty_users = self.cleaned_data.get("faculty")
        for user in faculty_users:
            if user.user_type != "faculty":
                raise forms.ValidationError(f"用户 {user.username} 不是教师类型，无法分配为授课教师")
        return faculty_users

    def clean_students(self):
        student_users = self.cleaned_data.get("students")
        for user in student_users:
            if user.user_type != "student":
                raise forms.ValidationError(f"用户 {user.username} 不是学生类型，无法分配为班级学生")
        return student_users

2. 在Admin中注册该表单

from django.contrib import admin
from .models import Classroom
from .forms import ClassroomForm

@admin.register(Classroom)
class ClassroomAdmin(admin.ModelAdmin):
    form = ClassroomForm

如果一定要在模型层校验，可先保存实例生成主键，再校验多对多关系，校验失败则回滚删除实例：

from django.db import transaction
from django.core.exceptions import ValidationError
from django.contrib.auth.models import User

class Classroom(models.Model):
    faculty = models.ManyToManyField(User, related_name="classrooms_as_faculty")
    students = models.ManyToManyField(User, related_name="classrooms_as_student")
    # 其他字段定义...

    def save(self, *args, **kwargs):
        is_new = self._state.adding
        # 先保存实例生成主键
        super().save(*args, **kwargs)
        if is_new:
            try:
                with transaction.atomic():
                    # 校验教师类型
                    invalid_faculty = self.faculty.exclude(user_type="faculty")
                    if invalid_faculty.exists():
                        raise ValidationError(f"无效教师用户：{', '.join([u.username for u in invalid_faculty])}")
                    # 校验学生类型
                    invalid_students = self.students.exclude(user_type="student")
                    if invalid_students.exists():
                        raise ValidationError(f"无效学生用户：{', '.join([u.username for u in invalid_students])}")
            except ValidationError as e:
                # 校验失败，删除已保存的实例
                self.delete()
                raise e

注意：这种方式会先创建实例再删除，可能产生短暂的无效数据，不如表单校验优雅。

内容的提问来源于stack exchange，提问作者Rajat Shenoi