gorm how to do insert with a has one relation that already exists

package main

import (
	"time"

	"gorm.io/driver/sqlite"
	"gorm.io/gorm"
	"gorm.io/gorm/logger"
)

type Country struct {
	ID        int
	Name      string `gorm:"uniqueIndex"`
	CreatedAt time.Time
	UpdatedAt time.Time
}
type Operator struct {
	ID        int
	CountryID int
	Country   Country
	Name      string
	Code      string
	CreatedAt time.Time
	UpdatedAt time.Time
}

func main() {
	config := gorm.Config{
		Logger: logger.Default.LogMode(logger.Info),
	}
	db, err := gorm.Open(sqlite.Open("test.db"), &config)

	if err != nil {
		panic(err)
	}

	db.AutoMigrate(&Country{}, &Operator{})
	test1 := Operator{Country: Country{Name: "Test"}, Name: "Test1", Code: "A"}
	db.Create(&test1)

	test2 := Operator{Country: Country{Name: "Test"}, Name: "Test2", Code: "B"}
	db.Create(&test2)

}

With the above code, test1 creates a new Country and Operator and the operator_id is correct in the xqlite database.

With test2 however, operator_id is set to 0. What should I do to create the test2 entry and have it refer to an existing Country ?

Your call to db.Create(&test2) is likely failing due to the uniqueIndex constraint on Country. Make sure to check your errors!

What you may find to work is to insert a single Country entry into the DB first, like so:

testCountry := Country{Name: "Test"}

if result := db.Create(&testCountry); result.Error != nil {
  // do something with result.Error
}

Based on my most recent experience working with Gorm, db.Create will populate testCountry.ID assuming it succeeds (always check your errors!). After that, create test1 and test2 like so:

test1 := Operator{
  Country:   testCountry,
  CountryID: testCountry.ID,
  Name:      "Test1", 
  Code:      "A",
}

if result := db.Create(&test1); result.Error != nil {
  // do something with result.Error
}

[... do the same thing with test2 ...]