django ORM多对多操作

创建多对多：
	方式一：自定义关系表
		class Host(models.Model):
			nid = models.AutoField(primary_key=True)
			hostname = models.CharField(max_length=32,db_index=True)
			ip = models.GenericIPAddressField(protocol="ipv4",db_index=True)
			port = models.IntegerField()
			b = models.ForeignKey(to="Business", to_field='id')
		# 10
		class Application(models.Model):
			name = models.CharField(max_length=32)
		# 2

		class HostToApp(models.Model):
			hobj = models.ForeignKey(to='Host',to_field='nid')
			aobj = models.ForeignKey(to='Application',to_field='id')

		# HostToApp.objects.create(hobj_id=1,aobj_id=2)

	方式二：自动创建关系表
		class Host(models.Model):
			nid = models.AutoField(primary_key=True)
			hostname = models.CharField(max_length=32,db_index=True)
			ip = models.GenericIPAddressField(protocol="ipv4",db_index=True)
			port = models.IntegerField()
			b = models.ForeignKey(to="Business", to_field='id')
		# 10
		class Application(models.Model):
			name = models.CharField(max_length=32)
			r = models.ManyToManyField("Host")

		无法直接对第三张表进行操作

		obj = Application.objects.get(id=1)
		obj.name

		# 第三张表操作
		obj.r.add(1)
		obj.r.add(2)
		obj.r.add(2,3,4)
		obj.r.add(*[1,2,3,4])

		obj.r.remove(1)
		obj.r.remove(2,4)
		obj.r.remove(*[1,2,3])

		obj.r.clear()

		# 所有相关的主机对象“列表” QuerySet
		obj.r.all()