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](https://docs.djangoproject.com/en/1.7/topics/i18n/timezones/) 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 http://127.0.0.1:8000/
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 http://127.0.0.1:8000/。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 *** 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 biggieit 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 wantthere'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
1. 创建项目运行面命令创建 django 项目项目名称叫 mysite ：
$ django-admin.py startproject mysite
创建项目目录：
mysite
├── manage.py
└── mysite
├── __init__.py
├── settings.py
├── urls.py
└── wsgi.py
1 directory, 5 files
说明：
__init__.py ：让 Python 该目录发包 (即组模块)所需文件 空文件般需要修改
manage.py ：种命令行工具允许种式与该 Django 项目进行交互 键入python manage.py help看能做 应需要编辑文件；目录纯便
settings.py ：该 Django 项目设置或配置
urls.py：Django项目URL路由设置目前空
wsgi.py：WSGI web 应用服务器配置文件更细节查看 How to deploy with WSGI
接修改 settings.py 文件例：修改 LANGUAGE_CODE、设置区 TIME_ZONE
SITE_ID = 1
LANGUAGE_CODE = 'zh_CN'
TIME_ZONE = 'Asia/Shanghai'
USE_TZ = True
面启 [Time zone]() 特性需要安装 pytz：
$ sudo pip install pytz
2. 运行项目
运行项目前我需要创建数据库表结构我使用默认数据库：
$ 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
启服务：
$ python manage.py runserver
看面输：
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.
端口8000启本服务器, 并且能台电脑连接访问 既服务器已经运行起现用网页浏览器访问 应该看令赏悦目淡蓝色 Django 欢迎页面始工作
指定启端口:
$ python manage.py runserver 8080
及指定 ip：
$ python manage.py runserver 0.0.0.0:8000
3. 创建 app
前面创建项目并且功运行现创建 app app 相于项目模块
项目目录创建 app：
$ python manage.py startapp polls
*** 作功 mysite 文件夹看已经叫 polls 文件夹目录结构：
polls
├── __init__.py
├── admin.py
├── migrations
│ └── __init__.py
├── models.py
├── tests.py
└── views.py
1 directory, 6 files
4. 创建模型
每 Django Model 都继承自 django.db.models.Model
Model 每属性 attribute 都代表 database field
通 Django Model API 执行数据库增删改查, 需要写些数据库查询语句
打 polls 文件夹 models.py 文件创建两模型：
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)
mysite/settings.py 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',
)
添加新 app 我需要运行面命令告诉 Django 模型做改变需要迁移数据库：
$ python manage.py makemigrations polls
看面输志：
Migrations for 'polls':
0001_initial.py:
- Create model Choice
- Create model Question
- Add field question to choice
polls/migrations/0001_initial.py 查看迁移语句
运行面语句查看迁移 sql 语句：
$ python manage.py sqlmigrate polls 0001
输结：
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
运行面命令检查数据库否问题：
$ python manage.py check
再运行面命令创建新添加模型：
$ python manage.py migrate
Operations to perform:
Apply all migrations: admin, contenttypes, polls, auth, sessions
Running migrations:
Applying polls.0001_initial... OK
总结修改模型需要做几步骤：
修改 models.py 文件
运行 python manage.py makemigrations 创建迁移语句
运行 python manage.py migrate模型改变迁移数据库
阅读 django-admin.py documentation查看更 manage.py 用
创建模型我通 Django 提供 API 做测试运行面命令进入 python shell 交互模式：
$ python manage.py shell
面些测试：
>>>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 biggieit 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=)
# 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()
[]
打印所 Question 输结 []我修改模型类使其输更易懂描述修改模型类：
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
接继续测试：
>>>from polls.models import Question, Choice
# Make sure our __str__() addition worked.
>>>Question.objects.all()
[]
# Django provides a rich database lookup API that's entirely driven by
# keyword arguments.
>>>Question.objects.filter(id=1)
[]
>>>Question.objects.filter(question_text__startswith='What')
[]
# 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)
# 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)
# 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)
>>>q.choice_set.create(choice_text='The sky', votes=0)
>>>c = q.choice_set.create(choice_text='Just hacking again', votes=0)
# Choice objects have API access to their related Question objects.
>>>c.question
# And vice versa: Question objects get access to Choice objects.
>>>q.choice_set.all()
[, , ]
>>>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 wantthere'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)
[, , ]
# Let's delete one of the choices. Use delete() for that.
>>>c = q.choice_set.filter(choice_text__startswith='Just hacking')
>>>c.delete()
>>>
面部测试涉及 django orm 相关知识详细说明参考 DjangoORM
5. 管理 admin
Django优秀特性, 内置Django admin台管理界面, 便管理者进行添加删除网站内容.
新建项目系统已经我设置台管理功能见 mysite/settings.py：
INSTALLED_APPS = (
'django.contrib.admin', #默认添加台管理功能
'django.contrib.auth',
'django.contrib.contenttypes',
'django.contrib.sessions',
'django.contrib.messages',
'django.contrib.staticfiles',
'mysite',
)
同已经添加进入台管理 url, mysite/urls.py 查看：
url(r'^admin/', include(admin.site.urls)), #使用设置url进入网站台
接我需要创建管理用户登录 admin 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
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
通面介绍 django 安装、运行及何创建视 图模型清晰认识接深入习 django 自化测试、持久化、间件、 际 化等知识
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 nullthis is only possible if null isTrue.
SET_DEFAULT [source] ¶
Set the ForeignKey to its default valuea 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.
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