Replacing an associated list on update

[johannes-riecken]

I'd like to replace an object in the database with the new one provided as function argument. In this example, each Pet has an associated list of Tags, but when I run .Save, the new Tags simply get appended. What's the recommended approach here? Should I add that approach as a section to gorm.io's pages/docs/update.md once I know about it?

For the example code below the actual output is:
ID: 1
Tags: [foo bar]
Whereas the desired output would be:
ID: 1
Tags: [bar]

package main

import (
	"fmt"
	"gorm.io/gorm"
	"log"
	"gorm.io/driver/postgres"
)

type Pet struct {
	gorm.Model
	Tags []Tag
}

type Tag struct {
	PetID int
	Name string `gorm:"primaryKey"`
}

func main() {
	err := mainAux()
	if err != nil {
		log.Fatal(err)
	}
}

func printPet(p *Pet) {
	fmt.Println("ID:", p.ID)
	var tags []string
	for _, x := range p.Tags {
		tags = append(tags, x.Name)
	}
	fmt.Println("Tags:", tags)
}

func mainAux() error {
	dsn := "host=localhost user=postgres password=changeme dbname=dbname port=5432"
	db, err := gorm.Open(postgres.New(postgres.Config{DSN: dsn}), &gorm.Config{})
	if err != nil {
		return fmt.Errorf("could not create postgres gorm: %w", err)
	}
	if err := db.Exec("DROP TABLE tags; DROP TABLE pets").Error; err != nil {
		return err
	}
	db.AutoMigrate(&Pet{}, &Tag{})
	if err := db.Create(&Pet{Tags: []Tag{{Name: "foo"}}}).Error; err != nil {
		return err
	}
	// // replace the Pet with ID 1
	if err := db.Save(&Pet{Model: gorm.Model{ID: 1}, Tags: []Tag{{Name: "bar"}}}).Error; err != nil {
		return err
	}
	p := &Pet{}
	if err := db.Preload("Tags").First(p).Error; err != nil {
		return err
	}
	printPet(p)

	return nil
}