你试图在尝试makemigrations时添加一个不可为空的字段（id）....

Question

这是发生了什么：

1. 我创建了以下模型： class Workout（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__

   class Activity（models.Model）：

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

   class CardioActivity（活动）：通过 class LiftActivity（Activity）：传递
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_cardio activity_lift activity_workout.朋友

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',),
        ),
    ]

3. 然后我把Activity类变成了元类： class Activity（models.Model）：

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

4. 并再次尝试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

这条消息的原因是什么？这对我没有任何意义。 CardioActivity自上次迁移以来甚至没有改变过，怎么能抱怨我试图添加一个id字段？

编辑：数据库中有什么？

当我检查数据库中的CardioActivity表时，这是我发现的：

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！然后Django怎么说它需要填充现有的行？

Answer 1

Django makemigrations不会检查表中是否有任何记录。因此，如果向模型添加新字段，则必须具有默认值，或者必须为该字段添加null = True