如何在django中存在多对多关系的情况下为现有模型添加额外的字段

from django.db import models

class A(models.Model):
    # 模型A的字段

class B(models.Model):
    # 模型B的字段

class AB(models.Model):
    a = models.ForeignKey(A, on_delete=models.CASCADE)
    b = models.ForeignKey(B, on_delete=models.CASCADE)
    extra_field = models.CharField(max_length=100)

class A(models.Model):
    b = models.ManyToManyField(B, through='AB')

class B(models.Model):
    a = models.ManyToManyField(A, through='AB')

a = A.objects.get(id=1)
b = B.objects.get(id=2)
ab = a.b.through.objects.get(a=a, b=b)
extra_field_value = ab.extra_field