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在做migrations的时候，偶尔会出现出错。
在已有数据的表中新增字段时，会弹出下面的信息
运行这个命令时
python manage.py makemigrations
Tracking file by folder pattern: migrations
It is impossible to add a non-nullable field ‘example’ to book without specifying a default. This is because the database needs something to populate existing rows.
Please select a fix:
- Provide a one-off default now (will be set on all existing rows with a null value for this column)
- Quit and manually define a default value in models.py.
Select an option:
解决方案：
选择1
之后会弹出
Please enter the default value as valid Python.
The datetime and django.utils.timezone modules are available, so it is possible to provide e.g. timezone.now as a value.
Type ‘exit’ to exit this prompt
输入, 必须加双引号
"1"
migrate 无法生成表
运行这个命令时
python manage.py migrate
会出现这种结果
Operations to perform:Apply all migrations: admin, app01, auth, contenttypes, sessions
Running migrations:No migrations to apply.
出现无法生成表的原因是，去到 django_migrations 表里查看，就能知道之前已经创建了表。所以得在该表用sql语句来删除对应的app数据。
最后运行下面两个命令
python manage.py makemigrationspython manage.py migrate
这样就可以创建表了。
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