Golang gorm manytomany 多对多 更新、删除、替换

Delete 移除 只删除中间表的数据   删除原有的

	var a Article1
	db.Preload("Tag1s").Take(&a, 1)
	fmt.Printf("%v", a)
{1 k8s [{1 cloud []} {2 linux []}]}

mysql> select * from article1;
+----+--------+
| id | title  |
+----+--------+
|  1 | k8s    |
|  2 | golang |
+----+--------+

mysql> select * from tag1;
+----+-------+
| id | name  |
+----+-------+
|  1 | cloud |
|  2 | linux |
+----+-------+

mysql> select * from article_tags;
+-------------+---------+
| article1_id | tag1_id |
+-------------+---------+
|           1 |       1 |
|           1 |       2 |
|           2 |       2 |
+-------------+---------+

（1）model 先拿到这张表，

（2）Association然后去连接tags第三张表

（3）之后就是最关键的要去执行什么操作，这里是delete，删除谁呢？删除的就是article关联的tag。

	var a Article1
	db.Preload("Tag1s").Take(&a, 1)
	db.Model(&a).Association("Tag1s").Delete(a.Tag1s)
	fmt.Printf("%v", a)

{1 k8s []}


mysql> select * from tag1;
+----+-------+
| id | name  |
+----+-------+
|  1 | cloud |
|  2 | linux |
+----+-------+

mysql> select * from article1;
+----+--------+
| id | title  |
+----+--------+
|  1 | k8s    |
|  2 | golang |
+----+--------+

mysql> select * from article_tags;
+-------------+---------+
| article1_id | tag1_id |
+-------------+---------+
|           2 |       2 |
+-------------+---------+

添加 和上面一样使用Append

	var t []Tag1
	db.Find(&t)

	var a Article1
	db.Preload("Tag1s").Take(&a, 1)
	db.Model(&a).Association("Tag1s").Append(&t)
	fmt.Printf("%v", a)

{1 k8s [{1 cloud []} {2 linux []}]}


mysql> select * from article1;
+----+--------+
| id | title  |
+----+--------+
|  1 | k8s    |
|  2 | golang |
+----+--------+

mysql> select * from tag1;
+----+-------+
| id | name  |
+----+-------+
|  1 | cloud |
|  2 | linux |
+----+-------+

mysql> select * from article_tags;
+-------------+---------+
| article1_id | tag1_id |
+-------------+---------+
|           1 |       1 |
|           1 |       2 |
|           2 |       2 |
+-------------+---------+

Replace  替换

	var t Tag1
	db.Take(&t, 1)

	var a Article1
	db.Preload("Tag1s").Take(&a, 2)
	db.Model(&a).Association("Tag1s").Replace(&t)
	fmt.Printf("%v", a)

{2 golang [{1 cloud []}]}