问题

我有一个名为`Car`的模型，它有一个`color`的CharField字段，可以选择`white`、`black`和`blue`。

```python
class Car(models.Model):

    WHITE = "White"
    BLACK = "Black"
    BLUE = "Blue"

    COLOR_CHOICES = (
        (WHITE, "White"),
        (BLACK, "Black"),
        (BLUE, "Blue"),
    )

    ...

    color = models.CharField(max_length=20, choices=COLOR_CHOICES, default=BLUE)

我已经创建了带有颜色的Car对象。现在，当我对选择进行更改（例如，在所有情况下将BLUE更改为RED），并运行迁移时，已经存在的颜色为BLUE的Car对象不会迁移到RED。这就是奇怪之处：

1. 当我使用django shell检查这些对象时，它们显示为Car.BLUE（不再存在的选择）。
2. 当我在Django管理界面中检查这些对象时，它们显示为Car.WHITE。
3. 当我创建一个新对象时，它可以正常工作 - 它会自动变成Car.RED（选择了默认值）。

问题：

• 在迁移CharField的选择时，是否有任何特定的步骤我忽略了？
• 为什么会出现这种奇怪的行为，如何安全地修复？
• 我是否需要手动（或通过脚本）修复数据？

我期望在迁移时，所有现有的Car.BLUE对象都会迁移到Car.RED，因为这是新的默认值。我尝试手动创建一个新对象，它会选择新的默认的RED颜色。我还尝试手动将现有对象从BLUE更改为RED，这也可以正常工作。应用迁移时没有出现错误。

Django版本 - 4.1.5
数据库 - PostgreSQL

<details>
<summary>英文:</summary>

I have a model `Car` with a CharField `color`, that has the choices of `white`, `black` and `blue`.

class Car(models.Model):

WHITE = "White"
BLACK = "Black"
BLUE = "Blue"

COLOR_CHOICES = (
    (WHITE, "White"),
    (BLACK, "Black"),
    (BLUE, "Blue"),
)

...

color = models.CharField(max_length=20, choices=COLOR_CHOICES, default=BLUE)

I already have created `Car` objects with a color. Now when I introduce a change to the choices (e.g. change BLUE to RED in all occurences, as well as in the default) and run the migrations, the `Car` objects of color BLUE that already exist **do not** get migrated to RED. And that's where the weirdness begins:
1. When I use the django shell to check the objects, they appear as Car.BLUE (the choice that no longer exists). 
2. When I inspect the objects in the Django Admin, they appear as Car.WHITE.
3. When I create a new object, it works - it becomes Car.RED automatically (picks up the default).

Questions:
- Are there any specific steps that I missed when migrating choices for the CharField?     
- Why could this weird behavior be happening and how can I safely fix?
- Do I have to manually (or through a script) fix the data?

I expect upon migration all existing Car.BLUE objects to migrate to Car.RED when I run the migrations, because this is the new default value. I tried to manually create a new object and it picks up the new default RED color. I also tried to manually change an existing object from BLUE to RED, which is also fine. There were no errors when applying the migrations.

*Django version - 4.1.5
DB - PostgreSQL*

</details>


# 答案1
**得分**: 0

Django如果您更改[**<code>choices=&hellip;</code>**参数](https://docs.djangoproject.com/en/stable/ref/models/fields/#choices)，实际上Django将进行迁移，实际上如果您更改任何参数，Django都将进行迁移。但是默认的迁移处理程序…在您更改`choices`时不会执行任何操作。可以实现不同的处理程序，但可能不值得努力。

这意味着最终不会执行任何数据库更改，因此我们必须自己执行这些操作。如果要修改数据，您可以创建[*数据迁移*](https://docs.djangoproject.com/en/stable/topics/migrations/#data-migrations)。数据迁移与普通迁移类似，只是我们编写数据库应如何更改的方式。

我们使用以下方式创建数据迁移：

```python manage.py <strong>makemigrations --empty</strong> <em>app_name</em>```

在迁移文件中，然后我们可以构建一个更新查询：

```python
from django.db import migrations


def <b>red_to_blue</b>(apps, schema_editor):
    Car = apps.get_model('<em>app_name</em>', 'Car')
    Car.objects.filter(color='Red')<b>.update(color='Blue')</b>


class Migration(migrations.Migration):
    dependencies = [
        ("<em>app_name</em>", "<em>1234_some_migration</em>"),
    ]

    operations = [
        migrations.RunPython(<b>red_to_blue</b>),
    ]

然后进行迁移，它将运行red_to_blue函数，并将数据库中的数据迁移。

operations = [
    migrations.RunPython(<b>red_to_blue</b>),
]</code></pre>