Why is a linked record in a "Belongs To" relation empty?-CodePudding

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
    Card   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",
        Card: 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.Card.Number)
}

// output
// name: john, number:

CodePudding user response：

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, Card 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("Card").Where(map[string]interface{}{"name": "john"}).First(&myUser)