英文:
Why is a linked record in a "Belongs To" relation empty?
问题
以下是一个基于GORM的程序，定义了两个数据库表（users和cards），其中用户拥有一张卡的逻辑。然后它创建数据库，填充数据，搜索并打印出其中唯一的记录。
我的问题是：在最后一次搜索中，卡片是空的，它没有与用户关联。
从数据库的角度来看，一切都没问题：
SELECT * FROM users
id name card_id
1 john 1
SELECT * FROM cards
id number
1 42
请注意，第一个结果中的card_id正确指向卡片的id。
为什么我的最后一次搜索返回一个空的卡片？
package main
import (
"fmt"
"gorm.io/driver/sqlite"
"gorm.io/gorm"
)
type User struct {
ID uint
Name string
UserCard Card
CardID uint
}
type Card struct {
ID uint
Number string
}
func main() {
// 初始化数据库
db, _ := gorm.Open(sqlite.Open("mytest.sqlite"), &gorm.Config{})
db.AutoMigrate(&User{}, &Card{})
// 创建一张卡片
db.Create(&Card{
Number: "42",
})
// 查找那张卡片
var myCard Card
db.Where(map[string]interface{}{"number": "42"}).First(&myCard)
// 创建一个带有那张卡片的用户
db.Create(&User{
Name: "john",
UserCard: myCard,
})
// 查找那个用户
var myUser User
db.Where(map[string]interface{}{"name": "john"}).First(&myUser)
// 打印他的名字和卡片号码
// 问题出在这里：卡片号码为空，就好像卡片没有关联起来一样
fmt.Printf("name: %v, number: %v", myUser.Name, myUser.UserCard.Number)
}
// 输出
// name: john, number:
英文:
Below is a program (based on GORM) that defines two database tables (users and cards) with the logic of a user having one card. It then creates the database, fills it in, searches and prints out the only record in it.
My problem: the card in the ultimate search is empty, it is not attached to the user.
From the database perspective everything is OK:
SELECT * FROM users
id name card_id
1 john 1
SELECT * FROM cards
id number
1 42
Please note that card_id in the first result correctly points to the id of the card.
Why is my last search returning an empty card?
package main
import (
"fmt"
"gorm.io/driver/sqlite"
"gorm.io/gorm"
)
type User struct {
ID uint
Name string
UserCard Card
CardID uint
}
type Card struct {
ID uint
Number string
}
func main() {
// initialize the database
db, _ := gorm.Open(sqlite.Open("mytest.sqlite"), &gorm.Config{})
db.AutoMigrate(&User{}, &Card{})
// create one card
db.Create(&Card{
Number: "42",
})
// find that card
var myCard Card
db.Where(map[string]interface{}{"number": "42"}).First(&myCard)
// create a user with that card
db.Create(&User{
Name: "john",
UserCard: myCard,
})
// find that user
var myUser User
db.Where(map[string]interface{}{"name": "john"}).First(&myUser)
// print his name and card number
// the problem is here: the card number is empty, as if the card was not linked
fmt.Printf("name: %v, number: %v", myUser.Name, myUser.UserCard.Number)
}
// output
// name: john, number:
答案1
得分: 1
go-gorm不会自动加载嵌套对象，你需要明确指定你想要加载嵌套对象。在你的情况下，UserCard是User的嵌套对象。你可以在这里看到如何使用Preload函数来实现，但大致上应该是这样的：
var myUser User
db.Preload("UserCard").Where(map[string]interface{}{"name": "john"}).First(&myUser)
英文:
go-gorm will not load nested objects out of the box and automatically, you will have to specify that you want nested objects loaded. In your case, UserCard is a nested object of User. Here you can see more details on how to do that with the Preload function, but it should look something like this:
var myUser User
db.Preload("UserCard").Where(map[string]interface{}{"name": "john"}).First(&myUser)
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