多对多关系

---

多对多关系，需要用第三张表存储两张表的关系

 多对多就必须得加上many2many的tag。article_tags是用来指定第三张表，

package main

import (
	"gorm.io/driver/mysql"
	"gorm.io/gorm"
)

type Tag struct {
	ID       uint
	Name     string
	Articles []Article `gorm:"many2many:articles_tags;"`
}

type Article struct {
	ID    uint
	Title string
	Tags  []Tag `gorm:"many2many:articles_tags;"`
}

func main() {
	dsn := "root:7PXjAkY!&nlR@tcp(192.168.11.128:3306)/test?charset=utf8mb4&parseTime=True&loc=Local"
	db, _ := gorm.Open(mysql.Open(dsn), &gorm.Config{})
	db.AutoMigrate(&Article{}, &Tag{})
	db.Create(&Article{
		Title: "python基础",
		Tags: []Tag{
			{Name: "python"},
			{Name: "后端"},
		},
	})
}

mysql> select * from articles;
+----+--------------+
| id | title        |
+----+--------------+
|  2 | python?虹?   |
+----+--------------+

mysql> select * from tags;
+----+--------+
| id | name   |
+----+--------+
|  1 | python |
|  2 | ?..   |
+----+--------+


mysql> select * from articles_tags;
+--------+------------+
| tag_id | article_id |
+--------+------------+
|      1 |          2 |
|      2 |          2 |
+--------+------------+

创建了一篇文章和两个tag，同时还会去创建其依赖关系。第三张表会自己去创建。

关联已有文章就得先查。

	var tags []Tag
	db.Find(&tags, "name in ?", []string{"python"})

	db.AutoMigrate(&Article{}, &Tag{})
	db.Create(&Article{
		Title: "golang",
		Tags:  tags,
	})



mysql> select * from articles_tags;
+--------+------------+
| tag_id | article_id |
+--------+------------+
|      1 |          2 |
|      2 |          2 |
|      1 |          4 |
+--------+------------+

mysql> select * from tags;
+----+--------+
| id | name   |
+----+--------+
|  1 | python |
|  2 | ?..   |
+----+--------+

mysql> select * from articles;
+----+--------------+
| id | title        |
+----+--------------+
|  2 | python?虹?   |
|  3 | golang       |
|  4 | golang       |
+----+--------------+