Question

这里发生了什么：

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__

班级CardioActivity（活动）：通

类LiftActivity（Activity）：通

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',),
        ),
    ]

然后我将Activity类转换为元类：

class Activity（models.Model）：

name = models.CharField(max_length = 100)

def __str__(self):
    return self.name
__repr__ = __str__

class Meta:
    abstract = True

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

此消息的原因是什么？这对我没有任何意义。自上次迁移以来，CardioActivity甚至没有被更改过，它如何抱怨我尝试添加id字段？

编辑：数据库中有什么？

当我检查数据库中的CardioActivity表时，我发现了以下内容：

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)

正如您所看到的，表中甚至没有任何现有的行！ Django怎么能说它需要填充现有的行？

Answer 1

Django makemigration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

您尝试在尝试进行迁移时尝试添加不可为空的字段（ID）

1 个答案: