django字段datetime set auto_now_add = true，但mysql error字段没有默认值

from django.db import models
from django.utils import timezone

class MyModel(models.Model):
    my_datetime_field = models.DateTimeField(default=timezone.now)

python manage.py makemigrations your_app_name

from django.db import migrations, models

def set_default_value(apps, schema_editor):
    MyModel = apps.get_model('your_app_name', 'MyModel')
    MyModel.objects.filter(my_datetime_field__isnull=True).update(my_datetime_field=timezone.now())

class Migration(migrations.Migration):

    dependencies = [
        ('your_app_name', '000x_previous_migration'),
    ]

    operations = [
        migrations.RunPython(set_default_value),
        migrations.AlterField(
            model_name='mymodel',
            name='my_datetime_field',
            field=models.DateTimeField(default=timezone.now),
        ),
    ]

python manage.py migrate your_app_name