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阿里云虚拟主机怎么建设网站,职业技能培训中心,广州网站设计服务商,北京室内设计公司排行榜GORM 指南 | GORM - The fantastic ORM library for Golang, aims to be developer friendly. 一对多入门比如要开发cmdb的系统，无论是硬件还是软件。硬件对应的就是对应的哪个开发在用。或者服务对应的是哪个业务模块在使用，或者应用谁在使用。那么这…

GORM 指南 | GORM - The fantastic ORM library for Golang, aims to be developer friendly.

一对多入门

比如要开发cmdb的系统，无论是硬件还是软件。硬件对应的就是对应的哪个开发在用。或者服务对应的是哪个业务模块在使用，或者应用谁在使用。那么这就是一对多的关系。

has many介绍

has many 关联就是创建和另一个模型的一对多关系（数据库里面是一对多，然后struct里面也是一对多）
例如，例如每一个用户都拥有多张信用卡，这样就是生活中一个简单的一对多关系

在设计表的时候不可能将所有的信息都放在一个表里面，那么表就会非常非常的宽。这样字段就会非常的多，性能就会受到影响。

在设计的时候这里其实就可以设置为两张表。一个是用户的详情表和信用卡的详情表，用户表里面加上卡的id和他做一个关联，那么这就是一对多的关系。

当一个数据库存储了很多数据的时候就需要分库分表，上面也类似，将一张表分为两张表。

creditcar的外键是userid。

package mainimport ("gorm.io/driver/mysql""gorm.io/gorm"
)type User struct {gorm.Model  //默认创建三个时间字段和一个id字段，create_at update_at delete_at这样可以知道创建时间/更新时间/删除时间CreditCards []*CreditCard66
}//用户有多张信用卡，UserID是外键
type CreditCard struct {gorm.ModelNumber   stringUserId int //默认会在CreditCard表中生成User66Id字段作为与user表关联的外键id//默认会在CreditCard表中生成UserID字段作为与User表关联的外键ID 
}func main() {dsn := "root:7PXjAkY!&nlR@tcp(192.168.11.128:3306)/test_db?charset=utf8mb4&parseTime=True&loc=Local"db, _ := gorm.Open(mysql.Open(dsn66), &gorm.Config{})db.AutoMigrate(User{}, CreditCard{})
}

mysql> show tables;
+-------------------+
| Tables_in_test_db |
+-------------------+
| credit_cards      |
| users             |
+-------------------+
2 rows in set (0.00 sec)mysql> desc users;
+------------+-----------------+------+-----+---------+----------------+
| Field      | Type            | Null | Key | Default | Extra          |
+------------+-----------------+------+-----+---------+----------------+
| id         | bigint unsigned | NO   | PRI | NULL    | auto_increment |
| created_at | datetime(3)     | YES  |     | NULL    |                |
| updated_at | datetime(3)     | YES  |     | NULL    |                |
| deleted_at | datetime(3)     | YES  | MUL | NULL    |                |
+------------+-----------------+------+-----+---------+----------------+mysql> desc credit_cards;
+------------+-----------------+------+-----+---------+----------------+
| Field      | Type            | Null | Key | Default | Extra          |
+------------+-----------------+------+-----+---------+----------------+
| id         | bigint unsigned | NO   | PRI | NULL    | auto_increment |
| created_at | datetime(3)     | YES  |     | NULL    |                |
| updated_at | datetime(3)     | YES  |     | NULL    |                |
| deleted_at | datetime(3)     | YES  | MUL | NULL    |                |
| number     | bigint          | YES  |     | NULL    |                |
| user_id    | bigint unsigned | YES  | MUL | NULL    |                |
+------------+-----------------+------+-----+---------+----------------+mysql> drop table users;
ERROR 3730 (HY000): Cannot drop table 'users' referenced by a foreign key constraint 'fk_users_credit_cards' on table 'credit_cards'.

二外键

为了定义一对多关系，外键是必须存在的，默认外键的名字是所有者类型的名字加上它的主键

(UserId) 。就像上面的例子，为了定义一个属于User 的模型，外键就应该为 UserID 。

使用其他的字段名作为外键，你可以通过 foreignkey 来定制它，例如 :

type User struct{gorm.ModelCreditCards []CreditCard `gorm:"foreignKey:UserRefer"`
}type CreditCard struct{gorm.ModelNumber  stringUserRefer uint
}

`gorm:"foreignKey:UserRefer"` 可以使用这个字段来去改它的外键。

import ("gorm.io/driver/mysql""gorm.io/gorm"
)type User struct {gorm.ModelCreditCards []*CreditCard `gorm:"foreignKey:UserRefer"`
}type CreditCard struct {gorm.ModelNumber    intUserRefer int  
}func main() {dsn6 := "root:7PXjAkY!&nlR@tcp(192.168.11.128:3306)/test_db?charset=utf8mb4&parseTime=True&loc=Local"db, _ := gorm.Open(mysql.Open(dsn6), &gorm.Config{})db.AutoMigrate(&User{}, &CreditCard{})
}mysql> desc users;
+------------+-----------------+------+-----+---------+----------------+
| Field      | Type            | Null | Key | Default | Extra          |
+------------+-----------------+------+-----+---------+----------------+
| id         | bigint unsigned | NO   | PRI | NULL    | auto_increment |
| created_at | datetime(3)     | YES  |     | NULL    |                |
| updated_at | datetime(3)     | YES  |     | NULL    |                |
| deleted_at | datetime(3)     | YES  | MUL | NULL    |                |
+------------+-----------------+------+-----+---------+----------------+mysql> desc credit_cards;
+------------+-----------------+------+-----+---------+----------------+
| Field      | Type            | Null | Key | Default | Extra          |
+------------+-----------------+------+-----+---------+----------------+
| id         | bigint unsigned | NO   | PRI | NULL    | auto_increment |
| created_at | datetime(3)     | YES  |     | NULL    |                |
| updated_at | datetime(3)     | YES  |     | NULL    |                |
| deleted_at | datetime(3)     | YES  | MUL | NULL    |                |
| number     | bigint          | YES  |     | NULL    |                |
| user_refer | bigint unsigned | YES  | MUL | NULL    |                |
+------------+-----------------+------+-----+---------+----------------+mysql> drop tables users;
ERROR 3730 (HY000): Cannot drop table 'users' referenced by a foreign key constraint 'fk_users_credit_cards' on table 'credit_cards'.

一般不推荐使用这种方式，还是推荐使用Userid的方式。

三外键关联

GORM 通常使用所有者的主键作为外键的值，在上面的例子中，它就是 User 的 ID 。当你分配信用卡给一个用户，GORM 将保存用户 ID 到信用卡表的 UserID 字段中。你能通过association_foreignkey来改变它。

type User struct {gorm.ModelMemberNumber string
// 默认CreditCard会使用User表的Id作为外键，association_foreignkey:MemberNumber
// 指定使用MemberNumber 作为外键关联
CreditCards []CreditCard
`gorm:"foreignkey:UserMemberNumber;association_foreignkey:MemberNumber"`
}type CreditCard struct {gorm.ModelNumber stringUserMemberNumber string
}

其实就是creditcard外键UserMemberNumber直接去找user的 MemberNumber。

上面其实也就是改变了外键的默认值，默认外键是Userid，那么现在变为了MemberNumber。

一般使用默认id的外键就能够覆盖很多场景，上面这种只是举例。

package mainimport ("gorm.io/driver/mysql""gorm.io/gorm"
)type User struct {gorm.ModelMemberNumber stringCreditCards  []*CreditCard `gorm:"foreignKey:UserMemberNumber;association_foreignKey:MemberNumber"`
}type CreditCard struct {gorm.ModelNumber           intUserMemberNumber string
}func main() {dsn6 := "root:7PXjAkY!&nlR@tcp(192.168.11.128:3306)/test_db?charset=utf8mb4&parseTime=True&loc=Local"db, _ := gorm.Open(mysql.Open(dsn6), &gorm.Config{})db.AutoMigrate(&User{}, &CreditCard{})
}mysql> desc users;
+---------------+-----------------+------+-----+---------+----------------+
| Field         | Type            | Null | Key | Default | Extra          |
+---------------+-----------------+------+-----+---------+----------------+
| id            | bigint unsigned | NO   | PRI | NULL    | auto_increment |
| created_at    | datetime(3)     | YES  |     | NULL    |                |
| updated_at    | datetime(3)     | YES  |     | NULL    |                |
| deleted_at    | datetime(3)     | YES  | MUL | NULL    |                |
| member_number | longtext        | YES  |     | NULL    |                |
+---------------+-----------------+------+-----+---------+----------------+mysql> desc credit_cards;
+--------------------+-----------------+------+-----+---------+----------------+
| Field              | Type            | Null | Key | Default | Extra          |
+--------------------+-----------------+------+-----+---------+----------------+
| id                 | bigint unsigned | NO   | PRI | NULL    | auto_increment |
| created_at         | datetime(3)     | YES  |     | NULL    |                |
| updated_at         | datetime(3)     | YES  |     | NULL    |                |
| deleted_at         | datetime(3)     | YES  | MUL | NULL    |                |
| number             | bigint          | YES  |     | NULL    |                |
| user_member_number | bigint unsigned | YES  | MUL | NULL    |                |
+--------------------+-----------------+------+-----+---------+----------------+

四创建一对多表

1. 表结构定义

/*
constraint:OnUpdate:CASCADE 【当User表更新，也会同步给CreditCards】 // 外键约束
OnDelete:SET NULL 【当User中数据被删除时，CreditCard关联设置为 NULL，不删除记录】
*/type User struct {gorm.ModelUsername string `json:"username" gorm:"column:username"`CreditCards []CreditCard `gorm:"constraint:OnUpdate:CASCADE,OnDelete:SET NULL;"`
}type CreditCard struct {gorm.ModelNumber stringUserID uint
}func main() {// 0、连接数据库dsn := "root:1@tcp(127.0.0.1:3306)/test_db?charset=utf8mb4&parseTime=True&loc=Local"db, _ := gorm.Open(mysql.Open(dsn), &gorm.Config{})// 创建表结构db.AutoMigrate(User{}, CreditCard{})// 1、创建一对多user := User{Username: "zhangsan",CreditCards: []CreditCard{{Number: "0001"},{Number: "0002"},},
}db.Create(&user)// 2、为已存在用户添加信用卡u := User{Username: "zhangsan"}db.First(&u)//fmt.Println(u.Username)}

上面如果要查询的话是没有去做关联查询的，只能查到user表相关的信息，并不能查到creditcard信息。

2. 创建结果说明

我们没有指定 foreignkey ，所以会与 UserID 字段自动建立外键关联关系

3. 一对多Association 查找关联使用 Association 方法, 需要把把 User 查询好, 然后根据 User 定义中指定的 AssociationForeignKey 去查找 CreditCard

import ("fmt""gorm.io/driver/mysql""gorm.io/gorm"
)type User struct {gorm.ModelUserName    string       `json:"username" gorm:"column:username"`CreditCards []CreditCard `gorm:"constraint:OnUpdate:CASCADE,OnDelete:SET NULL;"`
}type CreditCard struct {gorm.ModelNumber stringUserID int
}func main() {dsn6 := "root:7PXjAkY!&nlR@tcp(192.168.11.128:3306)/test_db?charset=utf8mb4&parseTime=True&loc=Local"db, _ := gorm.Open(mysql.Open(dsn6), &gorm.Config{})db.AutoMigrate(&User{}, &CreditCard{})/*u := &User{Model:    gorm.Model{},UserName: "lucas",CreditCards: []*CreditCard{{Number: "0001", UserID: 1},{Number: "0002", UserID: 2},},}db.Create(u)*/// 1、查找 用户名为 lucas 的所有信用卡信息u1 := &User{UserName: "lucas"}// Association必须要先查出User才能关联查询对应的CreditCarddb.First(u1)db.Model(u1).Association("CreditCards").Find(&u1.CreditCards)fmt.Println(u1)}

追加

package mainimport ("encoding/json""fmt""gorm.io/driver/mysql""gorm.io/gorm"
)type User struct {gorm.ModelUserName    string       `json:"username" gorm:"column:username"`CreditCards []CreditCard `gorm:"constraint:OnUpdate:CASCADE,OnDelete:SET NULL;"`
}type CreditCard struct {gorm.ModelNumber stringUserID int
}func main() {dsn6 := "root:7PXjAkY!&nlR@tcp(192.168.11.128:3306)/test_db?charset=utf8mb4&parseTime=True&loc=Local"db, _ := gorm.Open(mysql.Open(dsn6), &gorm.Config{})db.AutoMigrate(&User{}, &CreditCard{})u := &User{Model:    gorm.Model{},UserName: "lucas",CreditCards: []CreditCard{{Number: "0001", UserID: 0},{Number: "0002", UserID: 1},},}db.Create(u)u1 := &User{UserName: "lucas"}db.First(u1)db.Model(u1).Association("CreditCards").Find(&u1.CreditCards)fmt.Println(u1)db.Model(u1).Association("CreditCards").Append([]CreditCard{{Number: "0008", UserID: 1},})fmt.Println(u1)strUser, _ := json.Marshal(u1)fmt.Println(string(strUser))
}&{{1 2023-04-16 10:48:54.423 +0800 CST 2023-04-16 14:12:45.825 +0800 CST {0001-01-01 00:00:00 +0000 UTC false}} lucas []}&{{1 2023-04-16 10:48:54.423 +0800 CST 2023-04-16 14:15:51.44 +0800 CST {0001-01-01 00:00:00 +0000 UTC false}} lucas [{{17 2023-04-16 14:15:51.445+0800 CST 2023-04-16 14:15:51.445 +0800 CST {0001-01-01 00:00:00 +0000 UTC false}} 0008 1}]}{"ID":1,"CreatedAt":"2023-04-16T10:48:54.423+08:00","UpdatedAt":"2023-04-16T14:15:51.44+08:00","DeletedAt":null,"username":"lucas","CreditCards":[
{"ID":17,"CreatedAt":"2023-04-16T14:15:51.445+08:00","UpdatedAt":"2023-04-16T14:15:51.445+08:00","DeletedAt":null,"Number":"0008","UserID":1}]}

4. 一对多Preload 预加载

使用 Preload 方法, 在查询 User 时先去获取 CreditCard 的记录

和上面associate不一样，他们两个最大的区别是执行的顺序是不一样的，associate是先去获取user，再去获取creditcard。

preload是先去获取creditcard，再去获取user。

package main
import (
"encoding/json"
"fmt"
"gorm.io/driver/mysql"
"gorm.io/gorm"
)
/*
constraint:OnUpdate:CASCADE 【当User表更新，也会同步给CreditCards】
OnDelete:SET NULL 【当User中数据被删除时，CreditCard关联设置为 NULL，不删除记录】
*/
type User struct {
gorm.Model
Username string `json:"username" gorm:"column:username"`
CreditCards []CreditCard `gorm:"constraint:OnUpdate:CASCADE,OnDelete:SET
NULL;"`
}
type CreditCard struct {
gorm.Model
Number string
UserID uint
}
func main() {
// 0、连接数据库
dsn := "root:1@tcp(127.0.0.1:3306)/test_db?
charset=utf8mb4&parseTime=True&loc=Local"
db, _ := gorm.Open(mysql.Open(dsn), &gorm.Config{})
// 1、预加载: 查找 user 时预加载相关 CreditCards
//users := User{Username: "zhangsan"} // 只查找张三用户的信用卡信息
users := []User{}
db.Preload("CreditCards").Find(&users)
查询结果[
{
"ID":1,
"username":"zhangsan",
"CreditCards":[
{
"ID":1,
"Number":"0001",
"UserID":1
},
...
]
}
]