gorm 三表关联该如何使用？

有三张表来描述事件订阅

type EventSubscribe struct {
	gorm.Model
	Username      string          `gorm:"type:varchar(100)" json:"username"`
	EventTypes    []EventType     `gorm:"many2many:sub_event_tpyes;"`
	SubscribeType []SubscribeType `gorm:"many2many:sub_sub_types;"`
}

type EventType struct {
	gorm.Model
	Name string `gorm:"type:varchar(100)" json:"name"`
}

type SubscribeType struct {
	gorm.Model
	Name string `gorm:"type:varchar(100)" json:"name"`
}

我希望生成一张中间表

CREATE TABLE "sub_subType_eventType" (
	"sub_id" integer,
        "sub_type_id" integer,
	"event_type_id" integer,
	PRIMARY KEY ("sub_id","sub_type_id,event_type_id")
);

但是按照上面的方式建表是生成的两张关联表

求大神们指教！