从多表继承模型迁移到Django中的抽象基类

Question

我当前的项目使用multi-table inheritance models

from django.db import models

class Place(models.Model):
    name = models.CharField(max_length=50)
    address = models.CharField(max_length=80)

class Restaurant(Place):
    serves_hot_dogs = models.BooleanField(default=False)
    serves_pizza = models.BooleanField(default=False)

class Cinema(Place):
    sells_tickets = models.BooleanField(default=False)
    sells_popcorn = models.BooleanField(default=False)

我想换成abstract base classes。由于我的模型已经部署，我需要编写一些自定义迁移来将上面的模式转换为这个模式：

from django.db import models

class AbstractPlace(models.Model):
    name = models.CharField(max_length=50)
    address = models.CharField(max_length=80)

    class Meta:
        abstract = True

class Restaurant(AbstractPlace):
    serves_hot_dogs = models.BooleanField(default=False)
    serves_pizza = models.BooleanField(default=False)

class Cinema(AbstractPlace):
    sells_tickets = models.BooleanField(default=False)
    sells_popcorn = models.BooleanField(default=False)

有人对实现这一目标的步骤有什么建议吗？

Answer 1

我最近解决了这个问题，我通过在下面的代码块中编写和运行迁移解决了这个问题-松散地转换为适合您的情况的模型。

我非常确定不可能直接更改旧的Restaurant和Cinema模型的表，因为如果您尝试向它们添加字段，它们将与基本模型的现有字段发生冲突，并且如果您试图通过手动设置基本模型的options中的abstract=True来“解耦”派生模型和基本模型，Django报告说它无法找到Restaurant和Cinema的基本模型。(据我所知，这些问题可能是由bug引起的。)为了避免这个问题，我为派生模型创建了新的表，将数据从旧的表复制到新的表中，删除旧的表，并重命名新的表以匹配旧的表的名称。

我从Django生成的代码中获得了下面的大部分代码，可以通过创建一个临时迁移(在使用下面的代码创建一个迁移之前)，运行makemigrations，并从生成的迁移中复制CreateModel()和AlterField()(用于指向Restaurant或Cinema的相关字段)，来复制这些代码。

根据记录，我使用的是Django 3.1.4。

from django.db import migrations, models


def copy_objects_from_restaurant_and_cinema_to_restaurant_tmp_and_cinema_tmp(apps, schema_editor):
    Restaurant_Tmp = apps.get_model('<app name>', 'Restaurant_Tmp')
    Cinema_Tmp = apps.get_model('<app name>', 'Cinema_Tmp')
    Restaurant = apps.get_model('<app name>', 'Restaurant')
    Cinema = apps.get_model('<app name>', 'Cinema')
    # The `_meta.fields` list includes the PK
    copy_objects_from_old_model_to_new_model(Restaurant, Restaurant_Tmp, Restaurant_Tmp._meta.fields)
    copy_objects_from_old_model_to_new_model(Cinema, Cinema_Tmp, Cinema_Tmp._meta.fields)


def copy_objects_from_old_model_to_new_model(old_model, new_model, fields_to_copy):
    field_names = [field.name for field in fields_to_copy]
    for old_obj in old_model.objects.all():
        old_obj_field_dict = {
            field_name: getattr(old_obj, field_name)
            for field_name in field_names
        }
        new_model.objects.create(**old_obj_field_dict)


def copy_objects_from_restaurant_tmp_and_cinema_tmp_to_restaurant_and_cinema(apps, schema_editor):
    Restaurant_Tmp = apps.get_model('<app name>', 'Restaurant_Tmp')
    Cinema_Tmp = apps.get_model('<app name>', 'Cinema_Tmp')
    Restaurant = apps.get_model('<app name>', 'Restaurant')
    Cinema = apps.get_model('<app name>', 'Cinema')
    copy_objects_from_old_model_to_new_model(Restaurant_Tmp, Restaurant, Restaurant_Tmp._meta.fields)
    copy_objects_from_old_model_to_new_model(Cinema_Tmp, Cinema, Cinema_Tmp._meta.fields)


class Migration(migrations.Migration):

    dependencies = [
        ('<app name>', '<last migration>'),
    ]

    operations = [
        migrations.CreateModel(
            name='Restaurant_Tmp',
            fields=[
                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
                ('name', models.CharField(max_length=50)),
                ('address', models.CharField(max_length=80)),
                ('serves_hot_dogs', models.BooleanField(default=False)),
                ('serves_pizza', models.BooleanField(default=False)),
            ],
            options={
                'abstract': False,
            },
        ),
        migrations.CreateModel(
            name='Cinema_Tmp',
            fields=[
                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
                ('name', models.CharField(max_length=50)),
                ('address', models.CharField(max_length=80)),
                ('sells_tickets', models.BooleanField(default=False)),
                ('sells_popcorn', models.BooleanField(default=False)),
            ],
            options={
                'abstract': False,
            },
        ),
        migrations.RunPython(copy_objects_from_restaurant_and_cinema_to_restaurant_tmp_and_cinema_tmp, migrations.RunPython.noop),

        # Update foreign keys to reference the non-abstract models directly,
        # instead of through the (automatically generated) `place_ptr` field of the old models
        < 
          Run `migrations.AlterField()` here for each related field (like ForeignKey) of other models that point to Restaurant or Cinema,
          but change their `to` argument from e.g. `<app name>.restaurant` to `<app name>.restaurant_tmp`
        >
        migrations.RunPython(migrations.RunPython.noop, copy_objects_from_restaurant_tmp_and_cinema_tmp_to_restaurant_and_cinema),

        migrations.DeleteModel(
            name='Restaurant',
        ),
        migrations.DeleteModel(
            name='Cinema',
        ),
        migrations.DeleteModel(
            name='Place',
        ),
        migrations.RenameModel(
            old_name='Restaurant_Tmp',
            new_name='Restaurant',
        ),
        migrations.RenameModel(
            old_name='Cinema_Tmp',
            new_name='Cinema',
        ),
    ]

请注意，我最初编写的迁移只在使用SQLite的情况下进行了测试；其他数据库管理系统可能无法接受如此多的迁移操作，您可能必须将其拆分成多个迁移。(我有点不确定到底是什么导致了这个问题，但我记得我在使用PostgreSQL时就遇到过这种情况。)

如果这解决了你的问题，请让我知道！?