makemigrations报错

有关middleware的错误

提出问题

在运行python manage.py makemigrations出现了错误，错误信息如下：

ERRORS:
?: (admin.E408) 'django.contrib.auth.middleware.AuthenticationMiddleware' must be in MIDDLEWARE in order to use the admin application.
?: (admin.E409) 'django.contrib.messages.middleware.MessageMiddleware' must be in MIDDLEWARE in order to use the admin application.
?: (admin.E410) 'django.contrib.sessions.middleware.SessionMiddleware' must be in MIDDLEWARE in order to use the admin application.

解决方法

之前我将MIDDLERWARE的内容全部注释掉了，然后加上了自己想要修改的内容，由于报错，因此我将上面三个取消注释，最后结果如下：

MIDDLEWARE = [

    #'django.middleware.security.SecurityMiddleware',
    'django.contrib.sessions.middleware.SessionMiddleware',
    #'django.middleware.common.CommonMiddleware',
    #'django.middleware.csrf.CsrfViewMiddleware',
    'django.contrib.auth.middleware.AuthenticationMiddleware',
    'django.contrib.messages.middleware.MessageMiddleware',
    #'django.middleware.clickjacking.XFrameOptionsMiddleware',

    'django.contrib.admin',
    'django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
    'django.contrib.messages',
    'django.contrib.staticfiles',
    'myblog', #新建立的app
]

然后再尝试命令：
python manage.py makemigrations
成功运行。

有关model默认值的问题

提出问题

我之前的Shoes类是这样的：

class Shoes(models.Model):
    # 球鞋表的创建

    # 把id作为主键，考虑到后面网址的拼接id是str类型，
    # 所以这里主键设置为charfiled
    ShoesId=models.CharField(verbose_name='球鞋id',max_length=100,primary_key=True) #不要加default属性
    title=models.CharField(verbose_name='球鞋title',max_length=100)
    user=models.CharField(verbose_name='用户名称',max_length=100,default='root')
    class Meta:
        verbose_name = '球鞋id'
        verbose_name_plural = verbose_name

    def _str_(self):
        return self.title

后来添加了一个属性叫number：
number=models.CharField(verbose_name='球鞋货号',max_length=100)
然后运行命令：python manage.py makemigrations
这个报出错误：

You are trying to add a non-nullable field 'number' to shoes without a default; we can't do that (the database needs something to populate existing rows).
Please select a fix:
1)Provide a one-off default now (will be set on all existing rows with a null value for this column)
2)Quit, and let me add a default in models.py
Select an option:

解决办法

这种问题的解决办法一般是，将新增的属性设置一个默认值即可：
number=models.CharField(verbose_name='球鞋货号',max_length=100,default=" ")

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