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 usersid name card_id1 john 1SELECT * FROM cardsid number1 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 mainimport ("fmt""gorm.io/driver/sqlite""gorm.io/gorm")type User struct {ID uintName stringUserCard CardCardID uint}type Card struct {ID uintNumber 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 Carddb.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 Userdb.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 usersid name card_id1 john 1SELECT * FROM cardsid number1 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 mainimport ("fmt""gorm.io/driver/sqlite""gorm.io/gorm")type User struct {ID uintName stringUserCard CardCardID uint}type Card struct {ID uintNumber string}func main() {// initialize the databasedb, _ := gorm.Open(sqlite.Open("mytest.sqlite"), &gorm.Config{})db.AutoMigrate(&User{}, &Card{})// create one carddb.Create(&Card{Number: "42",})// find that cardvar myCard Carddb.Where(map[string]interface{}{"number": "42"}).First(&myCard)// create a user with that carddb.Create(&User{Name: "john",UserCard: myCard,})// find that uservar myUser Userdb.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 linkedfmt.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 Userdb.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 Userdb.Preload("UserCard").Where(map[string]interface{}{"name": "john"}).First(&myUser)
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