多对多中间表详解

我们都知道对于ManyToMany字段，Django采用的是第三张中间表的方式。通过这第三张表，来关联ManyToMany的双方。下面我们根据一个具体的例子，详细解说中间表的使用。

默认中间表

class Person(models.Model):
    name = models.CharField(max_length=128)

    def __str__(self):
        return self.name


class Group(models.Model):
    name = models.CharField(max_length=128)
    members = models.ManyToManyField(Person)

    def __str__(self):
        return self.name
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在Group模型中，通过members字段，以ManyToMany方式与Person模型建立了关系。
让我们来看看，中间表是个什么样子的：

首先有一列id，这是Django默认添加的，没什么好说的。然后是Group和Person的id列，这是默认情况下，Django关联两张表的方式。如果你要设置关联的列，可以使用to_field参数。

可见在中间表中，并不是将两张表的数据都保存在一起，而是通过id的关联进行映射。

通过through自定义中间表

一般情况，普通的多对多已经够用，无需自己创建第三张关系表。但是某些情况可能更复杂一点，比如如果你想保存某个人加入某个分组的时间呢？想保存进组的原因呢？

Django提供了一个through参数，用于指定中间模型，你可以将类似进组时间，邀请原因等其他字段放在这个中间模型内。例子如下：

modle:

from django.db import models


class Person(models.Model):
    name = models.CharField(max_length=128)

    def __str__(self):
        return self.name


class Group(models.Model):
    name = models.CharField(max_length=128)
    members = models.ManyToManyField(Person, through='Membership')

    def __str__(self):
        return self.name


class Membership(models.Model):
    person = models.ForeignKey(Person, on_delete=models.CASCADE)
    group = models.ForeignKey(Group, on_delete=models.CASCADE)
    date_joined = models.DateField()  # 进组时间
    invite_reason = models.CharField(max_length=64)  # 邀请原因
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view:

class PersonViews(ModelViewSet):

    queryset = Person.objects.filter()
    serializer_class = PersonSerializers


class GroupViews(ModelViewSet):

    queryset = Group.objects.filter()
    serializer_class = GroupSerializers


class MembershipViews(ModelViewSet):

    queryset = Membership.objects.filter()
    serializer_class = MembershipSerializers
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serializer:

from .models import Person, Group, Membership


class MembershipSerializers(serializers.ModelSerializer):

    class Meta:
        model = Membership
        fields = '__all__'


class PersonSerializers(serializers.ModelSerializer):

    class Meta:
        model = Person
        fields = '__all__'


class GroupSerializers(serializers.ModelSerializer):

    def to_representation(self, instance):
        representation = super(GroupSerializers, self).to_representation(instance)
        representation['members'] = []
        for i in PersonSerializers(instance.members, many=True).data:
            reason = MembershipSerializers(instance.membership_set.get(group=instance.id, person=i['id'])).data['invite_reason']
            i['invite_reason'] = reason
            representation['members'].append(i)
        return representation

    class Meta:
        model = Group
        fields = '__all__'

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reason = MembershipSerializers(instance.membership_set.get(group=instance.id, person=i[‘id’])).data[‘invite_reason’]
instance.membership_set.get(group=instance.id, person=i[‘id’]) group和person联合查出邀请原因
person和group模型上membership对象的默认名称都将为membership_set.所以通过instance.membership_set.get（）可以查看group下的所有关系

person下的所有membership:

    # def to_representation(self, instance):
    #     representation = super(PersonSerializers, self).to_representation(instance)
    #     representation['reason'] = MembershipSerializers(instance.membership_set, many=True).data
    #     return representation
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