django 在原有表格添加或删除字段的实例
一、如果models.py文件为时：
timestamp = models.DateTimeField('保存日期')
会提示：
(env8) D:\Desktop\env8\Scripts\mysite>python manage.py makemigrations You are trying to add a non-nullable field 'timestamp' to article without a defa ult; 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) 2) Quit, and let me add a default in models.py
输入：1 （这里要求你设置新建字段的默认值，它会在新建这个字段的同时把默认值也添加上去，）Select an option: 1
Please enter the default value now, as valid Python The datetime and django.utils.timezone modules are available, so you can do e.g. timezone.now() >>>
这里面不好修改
可以
(env8) D:\Desktop\env8\Scripts\mysite>python manage.py shell
(env8) D:\Desktop\env8\Scripts\mysite>from django.db import connection
(env8) D:\Desktop\env8\Scripts\mysite>cursor=connection.cursor()
(env8) D:\Desktop\env8\Scripts\mysite>cursor.execute('ALTER TABLEArticle add column timestamp varchar(100) default 0')
二、如果models.py文件为时：
timestamp = models.DateTimeField('保存日期',default=timezone.now,blank=False, null=False)
timestamp = models.DateTimeField('保存日期',default=timezone.now,blank=True, null=True)
blank
设置为True时，字段可以为空。设置为False时，字段是必须填写的。字符型字段CharField和TextField是用空字符串来存储空值的。如果为True，字段允许为空，默认不允许.
null
设置为True时，django用Null来存储空值。日期型、时间型和数字型字段不接受空字符串。所以设置IntegerField，DateTimeField型字段可以为空时，需要将blank，null均设为True。如果为True，空值将会被存储为NULL，默认为False。如果想设置BooleanField为空时可以选用NullBooleanField型字段。
(env8) D:\Desktop\env8\Scripts\mysite>python manage.py makemigrations就不会有下面的提示 (env8) D:\Desktop\env8\Scripts\mysite>python manage.py migrate 就行了中间不会设置数据类型（很容易出错）（若要设置默认值）
三、数据库设计是整个网站开发的核心
补充：timestamp = models.DateTimeField('保存日期')
(env8) D:\Desktop\env8\Scripts\mysite>python manage.py makemigrations You are trying to add a non-nullable field 'timestamp' to article without a defa ult; 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) 2) Quit, and let me add a default in models.py Select an option: 1 Please enter the default value now, as valid Python The datetime and django.utils.timezone modules are available, so you can do e.g. timezone.now() >>> '2017-12-16 05:04:31.000'（添加字段的数据类型格式） Migrations for 'blog': 0002_article_timestamp.py: - Add field timestamp to article (env8) D:\Desktop\env8\Scripts\mysite>python manage.py migrate Operations to perform: Synchronize unmigrated apps: staticfiles, ckeditor_uploader, messages, ckedito r, bootstrap3 Apply all migrations: admin, blog, contenttypes, auth, sessions Synchronizing apps without migrations: Creating tables... Running deferred SQL... Installing custom SQL... Running migrations: Rendering model states... DONE Applying blog.0002_article_timestamp...D:Desktop\env8\lib\site-packa ges\django\db\models\fields\__init__.py:1474: RuntimeWarning: DateTimeField Arti cle.timestamp received a naive datetime (2017-12-16 05:04:31) while time zone su pport is active. RuntimeWarning) OK (env8) D:\Desktop\env8\Scripts\mysite>
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