从多表继承模型迁移到Django中的抽象基类

Question

我当前的项目使用多表继承模型：

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)

我想改用抽象基类。由于我的模型已经部署，我需要编写一些自定义迁移以将上述架构转换为以下架构：

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)

有谁对实现这一目标所采取的步骤有任何建议吗？

Answer 1

我最近解决了这个确切的问题，我通过在下面的代码块中编写和运行迁移来解决这个问题 - 松散地翻译以适应您的案例中的模型。

我很确定不可能直接更改旧模型Restaurant和Cinema模型的表，就好像您尝试向其中添加字段一样，它们将与基本模型的现有字段发生冲突，并且如果您尝试“解耦”从基本模型派生的模型，例如通过abstract=True在基本模型的 中手动设置options，Django 报告它无法找到Restaurant和的基本模型Cinema。（据我所知，这些问题可能是由错误引起的。）为了避免这个问题，我为派生模型创建了新表，将数据从旧表复制到新表，删除旧表，并将新表与旧表的名称相匹配。

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之前）来复制这些代码，该临时迁移仅删除Restaurant、Cinema和、Place运行makemigrations以及复制CreateModel()s 和AlterField()s （对于相关字段）指向生成的迁移中的Restaurant或)。Cinema

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，我最初编写的迁移仅经过测试，可以使用 SQLite 进行工作；其他数据库管理系统可能不接受如此多的迁移操作，您可能必须将其拆分为多个迁移。（我有点不确定到底是什么导致了这个问题，但我记得我在 PostgreSQL 中遇到过这个问题。）

如果这解决了您的问题，请告诉我！