多对多--ManyToMany
应用场景
在某表中创建一行数据时，有一个可以多选的下拉框（一对一是单选框）
例如：创建用户信息，需要为用户指定多个爱好
创建表
两种方法，一个是利用Django自动为多对多创建关联的第三张表，另一种是自己手动创建关系表。
NO.1
根据Django中的代码，自动为多对多表创建一个第三张表对应关系
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class
Host(models.Model):
hostname
=
models.CharField(max_length
=
32
)
port
=
models.IntegerField()
class
HostAdmin(models.Model):
username
=
models.CharField(max_length
=
32
)
email
=
models.CharField(max_length
=
32
)
host
=
models.ManyToManyField(Host)
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NO.2
自定义多对多表，没有ORM都是自己生成
models.py
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class
Host1(models.Model):
hostname
=
models.CharField(max_length
=
32
)
port
=
models.IntegerField()
class
HostAdmin1(models.Model):
username
=
models.CharField(max_length
=
32
)
email
=
models.CharField(max_length
=
32
)
host
=
models.ManyToManyField(Host1, through
=
'HostRelation'
)
class
HostRelation(models.Model):
c1
=
models.ForeignKey(Host1)
c2
=
models.ForeignKey(HostAdmin1)
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view.py
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#多对多自定义创建表
models.HostRelation.objects.create(
c1
=
models.Host1.objects.get(
id
=
1
),
c2
=
models.HostAdmin1.objects.get(
id
=
2
)
)
models.HostRelation.objects.create(
c1_id
=
2
,
c2_id
=
1
)
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创建数据
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初始化数据
models.Host.objects.create(hostname
=
'c1'
,port
=
80
)
models.Host.objects.create(hostname
=
'c2'
,port
=
80
)
models.Host.objects.create(hostname
=
'c3'
,port
=
80
)
models.HostAdmin.objects.create(username
=
'root'
,email
=
'1@live.com'
)
models.HostAdmin.objects.create(username
=
'dali'
,email
=
'dali@live.com'
)
models.HostAdmin.objects.create(username
=
'haojie'
,email
=
'haojie@live.com'
)
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添加数据
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#正向添加
#目的，给大力分配两个主机的管理权限
#1、获取大力用户
admin_obj
=
models.HostAdmin.objects.get(username
=
'dali'
)
#2、获取指定的两个主机
host_list
=
models.Host.objects.
filter
(id__lt
=
3
)
#3、将两个主机和大力添加到第三张对应表中
admin_obj.host.add(
*
host_list)
#反向添加
host_obj
=
models.Host.objects.get(
id
=
3
)
admin_list
=
models.HostAdmin.objects.
filter
(id__gt
=
1
)
host_obj.hostadmin_set.add(
*
admin_list)
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查询数据
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#查询数据
#第一种方式
#正向查
admin_obj
=
models.HostAdmin.objects.
all
(
id
=
1
)
for
item
in
admin_obj:
item.host.
all
()
#反向查
host_obj
=
models.Host.objects.get(
id
=
1
)
host_obj.hostadmin_set.
all
()
#第二种方式
relation_list
=
models.HostRelation.objects.
all
()
relation_list
=
models.HostRelation.objects.
filter
(c2__username
=
'dali'
)
for
item
in
relation_list:
print
item.c1.hostname
print
item.c2.username
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