django - 如何使用 Django 1.7 迁移将现有的 django 模型迁移到 django-ordered-model？

Question

我有一个现有的模型，我想让它可以订购django-ordered-model，我选择它是因为它与django.contrib.admin.

现在该模型由 订购pk，我想保留该订购。当我运行manage.py makemigrations时它问我：

You are trying to add a non-nullable field 'orderable' to Item 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)
 2) Quit, and let me add a default in models.py

这两种选择似乎都是错误的。我应该怎么办？

Answer 1

我所做的是回答1，提供值0，然后在迁移文件中添加：

def set_initial_order(apps, schema_editor):
    model = apps.get_model('exercises', 'Item')
    for s in model.objects.all():
        s.order = s.pk
        s.save()

class Migration(migrations.Migration):
    # ...
    operations = [
        migrations.AlterModelOptions(
            # ...
        migrations.AddField(
            # ...
        migrations.RunPython(
            set_initial_order,
            reverse_code=lambda apps, schema_editor: None
        ),
    ]

这意味着顺序最初将为所有内容，但随后将立即更改为等于pk，并且在反向迁移中无需执行任何操作，因为该列即将被删除。

Answer 2

如果您有order_with_respect_to设置：

def set_initial_order(apps, schema_editor):
    Lecture = apps.get_model('lectures', 'Lecture')
    Item = apps.get_model('exercises', 'Item')
    for lecture in Lecture.objects.all():
        for index, item in enumerate(Item.objects.filter(lecture=lecture).order_by('id').all()):
            item.order = index
            item.save()


class Migration(migrations.Migration):

    # ...
    operations = [
        migrations.AlterModelOptions(
            # ...
        ),
        migrations.AddField(
            # ...
        ),
        migrations.RunPython(
            set_initial_order,
            reverse_code=lambda apps, schema_editor: None
        ),
    ]