ManyToManyField 双向关联

2018-01-07

Django ManyToManyField 字段数据关联

表结构

先看一段代码

class People(models.Model):
    user = models.ForeignKey(User, verbose_name='系统关联', on_delete=False, null=True, blank=True)
    birth_death = models.CharField('生卒', max_length=32, default='')
    name = models.CharField('姓名', max_length=64)
    address = models.CharField('地址', max_length=256, default='')
    desc = models.TextField('介绍', null=True, blank=True)
    childes = models.ManyToManyField('self', verbose_name='后辈')
    class Meta:
        verbose_name = '门人'

讨论的主要字段就一个childes

多对多数据，像多对多这个属性，在关系型数据库中的应用应该还是比较多的,平时使用应该比较少使用多对多自己的情况。然而我这里用到了，遇到一些问题，供大家参考

测试一下

p1 = People.objects.create(name='p1')
p2 = People.objects.create(name='p2')
p2.childes.add(p1)
p2.childes.all()  # <QuerySet [<People: People object (1)>]> ....... 1
p1.childes.all()  # <QuerySet [<People: People object (2)>]> ....... 2

看到....1的位置，发现我们成功添加一条数据到p2中，嗯，没毛病继续往下看，p1的数据，我们并没有往p1中添加数据，然而，这里却多了一条数据，这就是毛病了。

看看数据库

看到问题了吧？

id	from_people_id	to_people_id
1	2	1
2	1	2

看看官方文档

ManyToManyField.symmetrical¶
Only used in the definition of ManyToManyFields on self. Consider the following model:</p><p>from django.db import models</p><p>class Person(models.Model):
    friends = models.ManyToManyField("self")
When Django processes this model, it identifies that it has a ManyToManyField on itself, and as a result, it doesn’t add a person_set attribute to the Person class. Instead, the ManyToManyField is assumed to be symmetrical – that is, if I am your friend, then you are my friend.</p><p>If you do not want symmetry in many-to-many relationships with self, set symmetrical to False. This will force Django to add the descriptor for the reverse relationship, allowing ManyToManyField relationships to be non-symmetrical.

对了，看官应该看到了“symmetrical”对称的数据，布尔值

修改后的代码

1	childes = models.ManyToManyField('self', symmetrical=False, verbose_name='后辈')

python

测一下

1	p1.childes.all() # <QuerySet []>

没有数据了，只有一边数据了, 对，这就是我们要的结果

总结

ManyToManyField 字段，选项symmetrical 默认为True，双向关联，导致数据产生逆向关联。 ps: 有问题，找官方文档最靠谱