on_delete屬性針對(duì)外鍵ForeignKey

一、django3.0官方文檔介紹：

Many-to-one relationships多對(duì)一關(guān)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 modelsclass Manufacturer(models.Model): # ... passclass 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.

常見(jiàn)的使用方式(設(shè)置為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='請(qǐng)求關(guān)鍵字') response = models.TextField(verbose_name='響應(yīng)結(jié)果') api = models.ForeignKey(Api, blank=True, null=True, on_delete=models.SET_NULL, verbose_name='所屬接口') status = models.IntegerField(default=1, verbose_name='狀態(tài)') create_at = models.CharField(max_length=20, verbose_name='創(chuàng)建時(shí)間') update_at = models.CharField(max_length=20, verbose_name='更新時(shí)間')

一對(duì)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)

一對(duì)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)

從上面外鍵(ForeignKey)和一對(duì)一(OneToOneField)的參數(shù)中可以看出,都有on_delete參數(shù),而 django 升級(jí)到2.0之后,表與表之間關(guān)聯(lián)的時(shí)候,必須要寫(xiě)on_delete參數(shù),否則會(huì)報(bào)異常:

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

因此,整理一下on_delete參數(shù)的各個(gè)值的含義:

on_delete=None,# 刪除關(guān)聯(lián)表中的數(shù)據(jù)時(shí),當(dāng)前表與其關(guān)聯(lián)的field的行為on_delete=models.CASCADE, # 刪除關(guān)聯(lián)數(shù)據(jù),與之關(guān)聯(lián)也刪除on_delete=models.DO_NOTHING, # 刪除關(guān)聯(lián)數(shù)據(jù),什么也不做on_delete=models.PROTECT, # 刪除關(guān)聯(lián)數(shù)據(jù),引發(fā)錯(cuò)誤ProtectedError# models.ForeignKey(’關(guān)聯(lián)表’, on_delete=models.SET_NULL, blank=True, null=True)on_delete=models.SET_NULL, # 刪除關(guān)聯(lián)數(shù)據(jù),與之關(guān)聯(lián)的值設(shè)置為null（前提FK字段需要設(shè)置為可空,一對(duì)一同理）# models.ForeignKey(’關(guān)聯(lián)表’, on_delete=models.SET_DEFAULT, default=’默認(rèn)值’)on_delete=models.SET_DEFAULT, # 刪除關(guān)聯(lián)數(shù)據(jù),與之關(guān)聯(lián)的值設(shè)置為默認(rèn)值（前提FK字段需要設(shè)置默認(rèn)值,一對(duì)一同理）on_delete=models.SET, # 刪除關(guān)聯(lián)數(shù)據(jù), a. 與之關(guān)聯(lián)的值設(shè)置為指定值,設(shè)置：models.SET(值) b. 與之關(guān)聯(lián)的值設(shè)置為可執(zhí)行對(duì)象的返回值,設(shè)置：models.SET(可執(zhí)行對(duì)象)

多對(duì)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?yàn)槎鄬?duì)多(ManyToManyField)沒(méi)有 on_delete 參數(shù),所以略過(guò)不提.

二、on_delete外鍵刪除方式

CASCADE：級(jí)聯(lián)刪除。當(dāng)Manufacturer對(duì)象刪除時(shí)，它對(duì)應(yīng)的Car對(duì)象也會(huì)刪除。 PROTECT：保護(hù)模式，采用該選項(xiàng)，刪除時(shí)會(huì)拋出ProtectedError錯(cuò)誤。 SET_NULL：置空模式，刪除的時(shí)候，外鍵字段被設(shè)置為空，前提就是blank=True, null=True,定義該字段的時(shí)候，允許為空。當(dāng)Manufacturer對(duì)象刪除時(shí)，它對(duì)應(yīng)的Car對(duì)象的manufacturer字段會(huì)置空，前提是null=True SET_DEFAULT：置默認(rèn)值，刪除的時(shí)候，外鍵字段設(shè)置為默認(rèn)值，所以定義外鍵的時(shí)候注意加上一個(gè)默認(rèn)值。 SET()：自定義一個(gè)值，該值當(dāng)然只能是對(duì)應(yīng)的實(shí)體了

django3.0關(guān)于models官方文檔地址：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

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