Django迁移RunPython不能调用模型方法

from django.apps import apps

def my_migration_function(apps, schema_editor):
    MyModel = apps.get_model('myapp', 'MyModel')
    # 现在可以调用MyModel的方法
    MyModel.my_method()

class Migration(migrations.Migration):
    dependencies = [
        ('myapp', '0001_initial'),
    ]

    operations = [
        migrations.RunPython(my_migration_function),
    ]

from django.db import migrations

def my_migration_function(apps, schema_editor):
    from myapp.models import MyModel
    # 现在可以调用MyModel的方法
    MyModel.my_method()

class Migration(migrations.Migration):
    dependencies = [
        ('myapp', '0001_initial'),
    ]

    operations = [
        migrations.RunPython(my_migration_function),
    ]