您正在尝试添加一个不可为空的字段 (id)... 尝试进行迁移时

Question

发生了什么事：

1. 我创建了以下模型：

   类锻炼（models.Model）：

   datetime = models.DateTimeField()
   user = models.ForeignKey(User, on_delete=models.CASCADE)
   lifts = fields.LiftsField()
   cardio = JSONField()
   
   def __str__(self):
       return str(self.datetime)+" "+self.user.email
   
   __repr__ = __str__
   

   类活动（models.Model）：

   name = models.CharField(max_length = 100)
   
   def __str__(self):
       return self.name
   __repr__ = __str__
   

   类 CardioActivity(Activity): 通过

   类 LiftActivity(Activity): 通过

2. 我跑了makemigrations 和migrate

   workoutcal/migrations/0002_activity_cardioactivity_liftactivity_workout.py - Create model Activity - Create model Workout - Create model CardioActivity - Create model LiftActivity

0002_activity_cardioactivity_liftactivity_workout.py

class Migration(migrations.Migration):

    dependencies = [
        ('workoutcal', '0001_initial'),
    ]

    operations = [
        migrations.CreateModel(
            name='Activity',
            fields=[
                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
                ('name', models.CharField(max_length=100)),
            ],
        ),
        migrations.CreateModel(
            name='Workout',
            fields=[
                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
                ('datetime', models.DateTimeField()),
                ('lifts', workoutcal.fields.LiftsField()),
                ('cardio', django.contrib.postgres.fields.jsonb.JSONField()),
                ('user', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, to=settings.AUTH_USER_MODEL)),
            ],
        ),
        migrations.CreateModel(
            name='CardioActivity',
            fields=[
                ('activity_ptr', models.OneToOneField(auto_created=True, on_delete=django.db.models.deletion.CASCADE, parent_link=True, primary_key=True, serialize=False, to='workoutcal.Activity')),
            ],
            bases=('workoutcal.activity',),
        ),
        migrations.CreateModel(
            name='LiftActivity',
            fields=[
                ('activity_ptr', models.OneToOneField(auto_created=True, on_delete=django.db.models.deletion.CASCADE, parent_link=True, primary_key=True, serialize=False, to='workoutcal.Activity')),
            ],
            bases=('workoutcal.activity',),
        ),
    ]

1. 然后我把Activity 类变成了一个元类：

   类活动（models.Model）：

   name = models.CharField(max_length = 100)
   
   def __str__(self):
       return self.name
   __repr__ = __str__
   
   class Meta:
       abstract = True
   

2. 又尝试makemigrations：

   (workout) Sahands-MBP:workout sahandzarrinkoub$ python manage.py makemigrations You are trying to add a non-nullable field 'id' to cardioactivity without a default; we can't do that (the database needs something to populate existing rows). Please select a fix: 1) Provide a one-off default now (will be set on all existing rows with a null value for this column) 2) Quit, and let me add a default in models.py

此消息的原因是什么？这对我来说没有任何意义。 CardioActivity 自上次迁移以来甚至没有更改，它怎么会抱怨我试图添加 id 字段？？

编辑：数据库中有什么？

当我检查数据库中的CardioActivity 表时，我发现：

workout=# \dt
                     List of relations
 Schema |               Name               | Type  | Owner 
--------+----------------------------------+-------+-------
 public | auth_group                       | table | admin
 public | auth_group_permissions           | table | admin
 public | auth_permission                  | table | admin
 public | django_admin_log                 | table | admin
 public | django_content_type              | table | admin
 public | django_migrations                | table | admin
 public | django_session                   | table | admin
 public | workoutcal_activity              | table | admin
 public | workoutcal_cardioactivity        | table | admin
 public | workoutcal_liftactivity          | table | admin
 public | workoutcal_user                  | table | admin
 public | workoutcal_user_groups           | table | admin
 public | workoutcal_user_user_permissions | table | admin
 public | workoutcal_workout               | table | admin
(14 rows)

workout=# SELECT * FROM workoutcal_cardioactivity;
 activity_ptr_id 
-----------------
(0 rows)

如您所见，表中甚至没有任何现有行！那么 Django 怎么能说它需要填充现有的行呢？

Answer 1

Django makemigration 不会检查表中是否有任何记录。因此，如果您将新字段添加到模型中，它必须具有默认值，或者您必须为该字段添加 null=True