为Django中现有条目上的ForeignKey字段提供默认值 [英] Provide a default for ForeignKey field on existing entries in Django

问题描述

我有这个模型：

class Movie(models.Model):
    # here are some fields for this model

我在其中添加了以下字段（数据库已经装有电影模型）： / p>

to which I added the following field (the database was already populated with Movie models):

user = models.ForeignKey(User, default=1)

我先运行了命令'makemigrations'，然后又执行了'migrate'：

I ran the commands 'makemigrations' and then 'migrate':

python manage.py makemigrations myapp
python manage.py migrate 

但它没有工作。我想做的是将 user字段添加到Movie对象中，并为数据库中所有现有的Movie对象为其提供默认值（在这种情况下，默认值为id = 1的User对象）。

我尝试的另一件事是不使用默认值，然后在运行 makemigrations命令时将其设置为默认值1（通过选择立即提供一次性默认值选项） ）。在这两种情况下，当我运行 migrate命令时，都会收到一个IntegrityError：

but it doesn't work. What I want to do is to add the 'user' field to Movie objects and provide a default for it for all the existing Movie objects in my database (in this case the default is the User object with id=1).
Another thing that I tried is to leave it without the default value and then, when i run the 'makemigrations' command, to give it the default value 1 (by selecting the "provide a one-off default now" option). In both cases, when I run the 'migrate' command I get an IntegrityError:

django.db.utils.IntegrityError: movies_movie__new.user_id may not be NULL

我还检查了数据库中已经存在的Users的ID，确保存在id = 1的用户并且确实存在。那为什么不起作用呢？谢谢。

I also checked the ids for the Users that are already in the database to make sure that a User with id=1 exists and it does. So why doesn't it work? Thank you.

推荐答案

如果添加 null = True，则可以：

If you add "null=True" it works:

user = models.ForeignKey(User, default=1, null=True)

然后

python manage.py makemigrations movies

Migrations for 'movies':
0003_auto_20150316_0959.py:
...
- Add field user to Movie
...

然后

python manage.py migrate movies

数据库应如下所示：

id|user_id|name
1|1|movie_name
2|1|movie_2
3|1|movie_3
...

所有空的user_ids的值为1。因此，您就完成了。

All the empty user_ids get a value of 1. So you're done.

现在，如果您需要将该字段设为必填字段，只需将模型更改回

Now, if you need to make that field required, just change the model back to

user = models.ForeignKey(User, default=1)

再次迁移
并再次迁移

make another migration and migrate again

这一次它将起作用，因为所有字段都具有user_id。

This time it will work since all fields have user_id.

;-）

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