怎么重载djangodelete(2023年最新整理)

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django 2.0外键处理

Django2.0里model外键和一对一的on_delete参数

在django2.0后，定义外键和一对一关系的时候需要加on_delete选项，此参数为了避免两个表里的数据不一致问题，不然会报错：

TypeError: __init__() missing 1 required positional argument: 'on_delete'

举例说明：

user=models.OneToOneField(User)

owner=models.ForeignKey(UserProfile)

需要改成：

user=models.OneToOneField(User,on_delete=models.CASCADE)          --在老版本这个参数（models.CASCADE）是默认值

owner=models.ForeignKey(UserProfile,on_delete=models.CASCADE)    --在老版本这个参数（models.CASCADE）是默认值

参数说明：

on_delete有CASCADE、PROTECT、SET_NULL、SET_DEFAULT、SET()五个可选择的值

CASCADE：此值设置，是级联删除。

PROTECT：此值设置，是会报完整性错误。

SET_NULL：此值设置，会把外键设置为null，前提是允许为null。

SET_DEFAULT：此值设置，会把设置为外键的默认值。

SET()：此值设置，会调用外面的值，可以是一个函数。

一般情况下使用CASCADE就可以了。

下面是官方文档说明：

ForeignKey accepts other arguments that define the details of how the relation works.

ForeignKey.on_delete ¶

When an object referenced by a ForeignKey is deleted, Django will emulate the behavior of the SQL constraint specified by the on_delete argument. For example, if you have a nullable ForeignKey and you want it to be set null when the referenced object is deleted:

user=models.ForeignKey(User,models.SET_NULL,blank=True,null=True,)

Deprecated since version 1.9: on_delete will become a required argument in Django 2.0. In older versions it defaults toCASCADE.

The possible values for on_delete are found in django.db.models :

CASCADE [source] ¶

Cascade deletes. Django emulates the behavior of the SQL constraint ON DELETE CASCADE and also deletes the object containing the ForeignKey.

PROTECT [source] ¶

Prevent deletion of the referenced object by raising ProtectedError , a subclass of django.db.IntegrityError .

SET_NULL [source] ¶

Set the ForeignKey null; this is only possible if null isTrue.

SET_DEFAULT [source] ¶

Set the ForeignKey to its default value; a default for the ForeignKey must be set.

SET() [source] ¶

Set the ForeignKey to the value passed to SET() , or if a callable is passed in, the result of calling it. In most cases, passing a callable will be necessary to avoid executing queries at the time your models.py is imported:

fromdjango.confimportsettingsfromdjango.contrib.authimportget_user_modelfromdjango.dbimportmodelsdefget_sentinel_user():returnget_user_model().objects.get_or_create(username='deleted')[0]classMyModel(models.Model):user=models.ForeignKey(settings.AUTH_USER_MODEL,on_delete=models.SET(get_sentinel_user),)

DO_NOTHING [source] ¶

Take no action. If your database backend enforces referential integrity, this will cause an IntegrityError unless you manually add an SQLONDELETEconstraint to the database field.

ForeignKey.limit_choices_to ¶

Sets a limit to the available choices for this field when this field is rendered using aModelFormor the admin (by default, all objects in the queryset are available to choose). Either a dictionary, a Q object, or a callable returning a dictionary or Q object can be used.

For example:

staff_member=models.ForeignKey(User,on_delete=models.CASCADE,limit_choices_to={'is_staff':True},)

causes the corresponding field on theModelFormto list onlyUsersthat haveis_staff=True. This may be helpful in the Django admin.

The callable form can be helpful, for instance, when used in conjunction with the Pythondatetimemodule to limit selections by date range. For example:

deflimit_pub_date_choices():return{'pub_date__lte':datetime.date.utcnow()}limit_choices_to=limit_pub_date_choices

Iflimit_choices_tois or returns a Qobject , which is useful for complex queries , then it will only have an effect on the choices available in the admin when the field is not listed in raw_id_fields in theModelAdminfor the model.

Note

If a callable is used forlimit_choices_to, it will be invoked every time a new form is instantiated. It may also be invoked when a model is validated, for example by management commands or the admin. The admin constructs querysets to validate its form inputs in various edge cases multiple times, so there is a possibility your callable may be invoked several times.

DJANGO问题，queryset的delete方法

你试试下面这中写法，看是不是会全部删除

from models import Entry

query = Entry.all()

entries =query.fetch(1)

db.delete(entries)

Django不级联删除问题，怎么解决

在不需要删除的一对一和外键中，添加on_delete=models.DO_NOTHING参数

去刘江的DJango教程看看把，免费的，而且项目实战很精彩。

如何解决Django 1.8在migrate时失败

1. 创建项目

运行下面命令就可以创建一个 django 项目，项目名称叫 mysite ：

$ django-admin.py startproject mysite

创建后的项目目录如下：

mysite

├── manage.py

└── mysite

├── __init__.py

├── settings.py

├── urls.py

└── wsgi.py

1 directory, 5 files

说明：

__init__.py ：让 Python 把该目录当成一个开发包 (即一组模块)所需的文件。 这是一个空文件，一般你不需要修改它。

manage.py ：一种命令行工具，允许你以多种方式与该 Django 项目进行交互。 键入python manage.py help，看一下它能做什么。 你应当不需要编辑这个文件；在这个目录下生成它纯是为了方便。

settings.py ：该 Django 项目的设置或配置。

urls.py：Django项目的URL路由设置。目前，它是空的。

wsgi.py：WSGI web 应用服务器的配置文件。更多细节，查看 How to deploy with WSGI

接下来，你可以修改 settings.py 文件，例如：修改 LANGUAGE_CODE、设置时区 TIME_ZONE

SITE_ID = 1

LANGUAGE_CODE = 'zh_CN'

TIME_ZONE = 'Asia/Shanghai'

USE_TZ = True

上面开启了 [Time zone]() 特性，需要安装 pytz：

$ sudo pip install pytz

2. 运行项目

在运行项目之前，我们需要创建数据库和表结构，这里我使用的默认数据库：

$ python manage.py migrate

Operations to perform:

Apply all migrations: admin, contenttypes, auth, sessions

Running migrations:

Applying contenttypes.0001_initial... OK

Applying auth.0001_initial... OK

Applying admin.0001_initial... OK

Applying sessions.0001_initial... OK

然后启动服务：

$ python manage.py runserver

你会看到下面的输出：

Performing system checks...

System check identified no issues (0 silenced).

January 28, 2015 - 02:08:33

Django version 1.7.1, using settings 'mysite.settings'

Starting development server at

Quit the server with CONTROL-C.

这将会在端口8000启动一个本地服务器, 并且只能从你的这台电脑连接和访问。 既然服务器已经运行起来了，现在用网页浏览器访问 。你应该可以看到一个令人赏心悦目的淡蓝色 Django 欢迎页面它开始工作了。

你也可以指定启动端口:

$ python manage.py runserver 8080

以及指定 ip：

$ python manage.py runserver 0.0.0.0:8000

3. 创建 app

前面创建了一个项目并且成功运行，现在来创建一个 app，一个 app 相当于项目的一个子模块。

在项目目录下创建一个 app：

$ python manage.py startapp polls

如果操作成功，你会在 mysite 文件夹下看到已经多了一个叫 polls 的文件夹，目录结构如下：

polls

├── __init__.py

├── admin.py

├── migrations

│ └── __init__.py

├── models.py

├── tests.py

└── views.py

1 directory, 6 files

4. 创建模型

每一个 Django Model 都继承自 django.db.models.Model

在 Model 当中每一个属性 attribute 都代表一个 database field

通过 Django Model API 可以执行数据库的增删改查, 而不需要写一些数据库的查询语句

打开 polls 文件夹下的 models.py 文件。创建两个模型：

import datetime

from django.db import models

from django.utils import timezone

class Question(models.Model):

question_text = models.CharField(max_length=200)

pub_date = models.DateTimeField('date published')

def was_published_recently(self):

return self.pub_date = timezone.now() - datetime.timedelta(days=1)

class Choice(models.Model):

question = models.ForeignKey(Question)

choice_text = models.CharField(max_length=200)

votes = models.IntegerField(default=0)

然后在 mysite/settings.py 中修改 INSTALLED_APPS 添加 polls：

INSTALLED_APPS = (

'django.contrib.admin',

'django.contrib.auth',

'django.contrib.contenttypes',

'django.contrib.sessions',

'django.contrib.messages',

'django.contrib.staticfiles',

'polls',

)

在添加了新的 app 之后，我们需要运行下面命令告诉 Django 你的模型做了改变，需要迁移数据库：

$ python manage.py makemigrations polls

你会看到下面的输出日志：

Migrations for 'polls':

0001_initial.py:

- Create model Choice

- Create model Question

- Add field question to choice

你可以从 polls/migrations/0001_initial.py 查看迁移语句。

运行下面语句，你可以查看迁移的 sql 语句：

$ python manage.py sqlmigrate polls 0001

输出结果：

BEGIN;

CREATE TABLE "polls_choice" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "choice_text" varchar(200) NOT NULL, "votes" integer NOT NULL);

CREATE TABLE "polls_question" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "question_text" varchar(200) NOT NULL, "pub_date" datetime NOT NULL);

CREATE TABLE "polls_choice__new" ("id" integer NOT NULL PRIMARY KEY AUTOINCREMENT, "choice_text" varchar(200) NOT NULL, "votes" integer NOT NULL, "question_id" integer NOT NULL REFERENCES "polls_question" ("id"));

INSERT INTO "polls_choice__new" ("choice_text", "votes", "id") SELECT "choice_text", "votes", "id" FROM "polls_choice";

DROP TABLE "polls_choice";

ALTER TABLE "polls_choice__new" RENAME TO "polls_choice";

CREATE INDEX polls_choice_7aa0f6ee ON "polls_choice" ("question_id");

COMMIT;

你可以运行下面命令，来检查数据库是否有问题：

$ python manage.py check

再次运行下面的命令，来创建新添加的模型：

$ python manage.py migrate

Operations to perform:

Apply all migrations: admin, contenttypes, polls, auth, sessions

Running migrations:

Applying polls.0001_initial... OK

总结一下，当修改一个模型时，需要做以下几个步骤：

修改 models.py 文件

运行 python manage.py makemigrations 创建迁移语句

运行 python manage.py migrate，将模型的改变迁移到数据库中

你可以阅读 django-admin.py documentation，查看更多 manage.py 的用法。

创建了模型之后，我们可以通过 Django 提供的 API 来做测试。运行下面命令可以进入到 python shell 的交互模式：

$ python manage.py shell

下面是一些测试：

from polls.models import Question, Choice # Import the model classes we just wrote.

# No questions are in the system yet.

Question.objects.all()

[]

# Create a new Question.

# Support for time zones is enabled in the default settings file, so

# Django expects a datetime with tzinfo for pub_date. Use timezone.now()

# instead of datetime.datetime.now() and it will do the right thing.

from django.utils import timezone

q = Question(question_text="What's new?", pub_date=timezone.now())

# Save the object into the database. You have to call save() explicitly.

q.save()

# Now it has an ID. Note that this might say "1L" instead of "1", depending

# on which database you're using. That's no biggie; it just means your

# database backend prefers to return integers as Python long integer

# objects.

q.id

1

# Access model field values via Python attributes.

q.question_text

"What's new?"

q.pub_date

datetime.datetime(2012, 2, 26, 13, 0, 0, 775217, tzinfo=UTC)

# Change values by changing the attributes, then calling save().

q.question_text = "What's up?"

q.save()

# objects.all() displays all the questions in the database.

Question.objects.all()

[Question: Question object]

打印所有的 Question 时，输出的结果是 [Question: Question object]，我们可以修改模型类，使其输出更为易懂的描述。修改模型类：

from django.db import models

class Question(models.Model):

# ...

def __str__(self): # __unicode__ on Python 2

return self.question_text

class Choice(models.Model):

# ...

def __str__(self): # __unicode__ on Python 2

return self.choice_text

接下来继续测试：

from polls.models import Question, Choice

# Make sure our __str__() addition worked.

Question.objects.all()

[Question: What's up?]

# Django provides a rich database lookup API that's entirely driven by

# keyword arguments.

Question.objects.filter(id=1)

[Question: What's up?]

Question.objects.filter(question_text__startswith='What')

[Question: What's up?]

# Get the question that was published this year.

from django.utils import timezone

current_year = timezone.now().year

Question.objects.get(pub_date__year=current_year)

Question: What's up?

# Request an ID that doesn't exist, this will raise an exception.

Question.objects.get(id=2)

Traceback (most recent call last):

...

DoesNotExist: Question matching query does not exist.

# Lookup by a primary key is the most common case, so Django provides a

# shortcut for primary-key exact lookups.

# The following is identical to Question.objects.get(id=1).

Question.objects.get(pk=1)

Question: What's up?

# Make sure our custom method worked.

q = Question.objects.get(pk=1)

# Give the Question a couple of Choices. The create call constructs a new

# Choice object, does the INSERT statement, adds the choice to the set

# of available choices and returns the new Choice object. Django creates

# a set to hold the "other side" of a ForeignKey relation

# (e.g. a question's choice) which can be accessed via the API.

q = Question.objects.get(pk=1)

# Display any choices from the related object set -- none so far.

q.choice_set.all()

[]

# Create three choices.

q.choice_set.create(choice_text='Not much', votes=0)

Choice: Not much

q.choice_set.create(choice_text='The sky', votes=0)

Choice: The sky

c = q.choice_set.create(choice_text='Just hacking again', votes=0)

# Choice objects have API access to their related Question objects.

c.question

Question: What's up?

# And vice versa: Question objects get access to Choice objects.

q.choice_set.all()

[Choice: Not much, Choice: The sky, Choice: Just hacking again]

q.choice_set.count()

3

# The API automatically follows relationships as far as you need.

# Use double underscores to separate relationships.

# This works as many levels deep as you want; there's no limit.

# Find all Choices for any question whose pub_date is in this year

# (reusing the 'current_year' variable we created above).

Choice.objects.filter(question__pub_date__year=current_year)

[Choice: Not much, Choice: The sky, Choice: Just hacking again]

# Let's delete one of the choices. Use delete() for that.

c = q.choice_set.filter(choice_text__startswith='Just hacking')

c.delete()

上面这部分测试，涉及到 django orm 相关的知识，详细说明可以参考 Django中的ORM。

5. 管理 admin

Django有一个优秀的特性, 内置了Django admin后台管理界面, 方便管理者进行添加和删除网站的内容.

新建的项目系统已经为我们设置好了后台管理功能，见 mysite/settings.py：

INSTALLED_APPS = (

'django.contrib.admin', #默认添加后台管理功能

'django.contrib.auth',

'django.contrib.contenttypes',

'django.contrib.sessions',

'django.contrib.messages',

'django.contrib.staticfiles',

'mysite',

)

同时也已经添加了进入后台管理的 url, 可以在 mysite/urls.py 中查看：

url(r'^admin/', include(admin.site.urls)), #可以使用设置好的url进入网站后台

接下来我们需要创建一个管理用户来登录 admin 后台管理界面：

$ python manage.py createsuperuser

Username (leave blank to use 'june'): admin

Email address:

Password:

Password (again):

Superuser created successfully.

总结

最后，来看项目目录结构：

mysite

├── db.sqlite3

├── manage.py

├── mysite

│ ├── __init__.py

│ ├── settings.py

│ ├── urls.py

│ ├── wsgi.py

├── polls

│ ├── __init__.py

│ ├── admin.py

│ ├── migrations

│ │ ├── 0001_initial.py

│ │ ├── __init__.py

│ ├── models.py

│ ├── templates

│ │ └── polls

│ │ ├── detail.html

│ │ ├── index.html

│ │ └── results.html

│ ├── tests.py

│ ├── urls.py

│ ├── views.py

└── templates

└── admin

└── base_site.htm

通过上面的介绍，对 django 的安装、运行以及如何创建视 图和模型有了一个清晰的认识，接下来就可以深入的学习 django 的自动化测试、持久化、中间件、国 际 化等知识。

Django models 多对一关系中 on_delete 参数的设置

Django 中使用 django.db.models.ForeignKey  定义多对一关系。

在定义中，有一个参数是必须要设置的，就是 on_delete。

顾名思义，就是设置在删除关联表中的数据时，当前表与其关联的field的行为。

改参数的取值有：

None：关联的数据同时删除；

models.CASCADE：与None类似，默认删除；

models.DO_NOTHING：什么都不做，不报错；

models.PROTECT：阻止删除，并触发ProtectError报错；

models.SET_NULL：设置当前表关联数据的值为null（必须支持null，即null参数为True）；

models.SET_DEFAULT：设置当前关联数据的值为default（必须定义了default）；

models.SET(a)：设置当前关联数据的值为a（a为一个具体的值或者一个全局可调用的回调函数）；

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