on_delete属性针对外键ForeignKey
一、django3.0官方文档介绍:
Many-to-one relationships多对一关系
To define a many-to-one relationship, use django.db.models.ForeignKey. You use it just like any other Field type: by including it as a class attribute of your model.
ForeignKey requires a positional argument: the class to which the model is related.
For example, if a Car model has a Manufacturer – that is, a Manufacturer makes multiple cars but each Car only has one Manufacturer – use the following definitions:
from django.db import models class Manufacturer(models.Model): # ... pass class Car(models.Model): manufacturer = models.ForeignKey(Manufacturer, on_delete=models.CASCADE) # ...
You can also create recursive relationships (an object with a many-to-one relationship to itself) and relationships to models not yet defined; see the model field reference for details.
It's suggested, but not required, that the name of a ForeignKey field (manufacturer in the example above) be the name of the model, lowercase. You can, of course, call the field whatever you want.
常见的使用方式(设置为null)
class ApiList(models.Model): desc = models.CharField(max_length=255, verbose_name="接口描述") keyword = models.CharField(max_length=100, verbose_name="请求关键字") response = models.TextField(verbose_name="响应结果") api = models.ForeignKey(Api, blank=True, null=True, on_delete=models.SET_NULL, verbose_name="所属接口") status = models.IntegerField(default=1, verbose_name="状态") create_at = models.CharField(max_length=20, verbose_name="创建时间") update_at = models.CharField(max_length=20, verbose_name="更新时间")
一对多(ForeignKey)
class ForeignKey(ForeignObject): def __init__(self, to, on_delete, related_name=None, related_query_name=None, limit_choices_to=None, parent_link=False, to_field=None, db_constraint=True, **kwargs): super().__init__(to, on_delete, from_fields=['self'], to_fields=[to_field], **kwargs)
一对一(OneToOneField)
class OneToOneField(ForeignKey): def __init__(self, to, on_delete, to_field=None, **kwargs): kwargs['unique'] = True super().__init__(to, on_delete, to_field=to_field, **kwargs)
从上面外键(ForeignKey)和一对一(OneToOneField)的参数中可以看出,都有on_delete参数,而 django 升级到2.0之后,表与表之间关联的时候,必须要写on_delete参数,否则会报异常:
TypeError: __init__() missing 1 required positional argument: 'on_delete'
因此,整理一下on_delete参数的各个值的含义:
on_delete=None, # 删除关联表中的数据时,当前表与其关联的field的行为 on_delete=models.CASCADE, # 删除关联数据,与之关联也删除 on_delete=models.DO_NOTHING, # 删除关联数据,什么也不做 on_delete=models.PROTECT, # 删除关联数据,引发错误ProtectedError # models.ForeignKey('关联表', on_delete=models.SET_NULL, blank=True, null=True) on_delete=models.SET_NULL, # 删除关联数据,与之关联的值设置为null(前提FK字段需要设置为可空,一对一同理) # models.ForeignKey('关联表', on_delete=models.SET_DEFAULT, default='默认值') on_delete=models.SET_DEFAULT, # 删除关联数据,与之关联的值设置为默认值(前提FK字段需要设置默认值,一对一同理) on_delete=models.SET, # 删除关联数据, a. 与之关联的值设置为指定值,设置:models.SET(值) b. 与之关联的值设置为可执行对象的返回值,设置:models.SET(可执行对象)
多对多(ManyToManyField)
class ManyToManyField(RelatedField): def __init__(self, to, related_name=None, related_query_name=None, limit_choices_to=None, symmetrical=None, through=None, through_fields=None, db_constraint=True, db_table=None, swappable=True, **kwargs): super().__init__(**kwargs)
因为多对多(ManyToManyField)没有 on_delete 参数,所以略过不提.
二、on_delete外键删除方式
- CASCADE:级联删除。当Manufacturer对象删除时,它对应的Car对象也会删除。
- PROTECT:保护模式,采用该选项,删除时会抛出ProtectedError错误。
- SET_NULL:置空模式,删除的时候,外键字段被设置为空,前提就是blank=True, null=True,定义该字段的时候,允许为空。当Manufacturer对象删除时,它对应的Car对象的manufacturer字段会置空,前提是null=True
- SET_DEFAULT:置默认值,删除的时候,外键字段设置为默认值,所以定义外键的时候注意加上一个默认值。
- SET():自定义一个值,该值当然只能是对应的实体了
django3.0关于models官方文档地址:
1.https://docs.djangoproject.com/en/3.0/topics/db/models/
2.https://docs.djangoproject.com/en/3.0/ref/models/fields/#django.db.models.ForeignKey
免责声明:本站资源来自互联网收集,仅供用于学习和交流,请遵循相关法律法规,本站一切资源不代表本站立场,如有侵权、后门、不妥请联系本站删除!
更新日志
- 纯音入心系列纯音乐《天籁古筝》1CD[MP3][331MB]
- 男女对唱典藏天碟《发烧对唱·那个季节里的歌DSD》10CD[WAV]
- 群星2010-歌林精选辑[歌林][WAV+CUE]
- TrioZimbalist-PianoTriosofWeinberg,Auerbach,Dvorak(2024)[24-44,1]wav
- 群星.1992-滚石第一流台湾歌·12王牌大车拼【滚石】【WAV+CUE】
- 林子祥.1984-创作歌集【华纳】【WAV+CUE】
- 周汤豪.2010-周汤豪【华纳】【FLAC分轨】
- Mozart-TheStringQuintets-AmadeusQuartet,CecilAronowitz(2017)[24-44,1][WAV+CUE]
- JamesWilliamsDennisIrwin-Focus(2024,Red)[24-48]FLAC
- 藤泽麻衣《空みあげて》[WAV]
- 纯音入心系列纯音乐《中国古筝经典名曲》1CD[MP3][1.3GB]
- 纯音入心系列纯音乐《古筝新奏:岁月静好与筝语,细水流年与筝同》1CD[MP3][846.9MB]
- 670《脆弱敏感小女生》[320K/MP3][27.53MB]
- 曾琳.2011-好好爱我【南方】【WAV+CUE】
- 范晓萱.1995-RAIN【福茂】【WAV+CUE】