迁移前从模型访问 Django 模型字段的问题

Question

我正在将字段 some_field 从 Model_A 移动到新的 Model_B，并具有 OneToOne 关系。在删除Model_A 中的此字段之前，我想将（历史）Model_A 中的值复制到新创建的Model_B。问题是我无法在迁移时检索该字段，因为Model_A 不再包含some_field。

这是我尝试运行自定义迁移时收到的错误消息：

AttributeError: 'Model_A' object has no attribute 'some_field'

更改前的模型：

class Model_A:
    some_field = models.BooleanField(default=False)
    some_other_field = models.BooleanField(default=False)

修改后的模型：

class Model_A:
    some_other_field = models.BooleanField(default=False)

class Model_B:
    model_a = models.OneToOneField(Model_A, related_name='extension')
    some_field = models.BooleanField(default=False)

迁移：

class Migration(migrations.Migration):

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

    def forwards_func(apps, schema_editor):
        # This is where I try to get the "historical" Model_A
        Model_A = apps2.get_model("my_app", "Model_A")

        # And this is where I intend to copy the some_field values
        for model_A_instance in Model_A.objects.all():
            b = Model_B(model_a=model_A_instance)
            # b gets created correctly, but the following step fails
            b.some_field = modelA_instance.some_field
            b.save()

    operations = [
        migrations.CreateModel(
            name='Model_B',
            fields=[
                ('id', models.AutoField(verbose_name='ID', serialize=False, auto_created=True, primary_key=True)),
                ('some_field', models.BooleanField(default=False)),
                ('model_a', models.OneToOneField(related_name='extension', to='my_app.Model_A')),
            ],
            options={
            },
            bases=(models.Model,),
        ),

        migrations.RunPython(forwards_func),

        migrations.RemoveField(
            model_name='model_a',
            name='some_field',
        ),
    ]

我非常清楚我必须以某种方式获取Model_A（=当前数据库中的那个）的“历史”表示，但我认为这就是apps2.get_model("my_app", "Model_A") 部分的用途。

关于如何实现这一点的任何意见？或者我应该将迁移一分为二，第一个创建 Model_B + 复制 some_field 值，第二个从 Model_A 中删除 some_field 字段？

Answer 1

是的，您需要有Model_A 的历史表示，并且很容易检索它。这就是apps 传递给RunPython 迁移调用的函数的目的，那么为什么你在这里使用一些apps2 insdead 的应用程序呢？此外，您应该从与 Model_A 相同的 apps 实例中获取您的 Model_B。您的迁移应如下所示：

class Migration(migrations.Migration):

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

    def forwards_func(apps, schema_editor):
        # This is where I try to get the "historical" Model_A
        # here is change - using apps passed into forwards_func by RunPython instead of apps2
        Model_A = apps.get_model("my_app", "Model_A")
        Model_B = apps.get_model("my_app", "Model_B")

        # And this is where I intend to copy the some_field values
        for model_A_instance in Model_A.objects.all():
            b = Model_B(model_a=model_A_instance)
            b.some_field = modelA_instance.some_field
            b.save()

    operations = [
        migrations.CreateModel(
            name='Model_B',
            fields=[
                ('id', models.AutoField(verbose_name='ID', serialize=False, auto_created=True, primary_key=True)),
                ('some_field', models.BooleanField(default=False)),
                ('model_a', models.OneToOneField(related_name='extension', to='my_app.Model_A')),
            ],
            options={
            },
            bases=(models.Model,),
        ),

        migrations.RunPython(forwards_func),

        migrations.RemoveField(
            model_name='model_a',
            name='some_field',
        ),
    ]

你为什么还要使用apps2？它是什么？