Duplicate entry when updating through parent of Has Many relationship

There is a "has many" relationship between Quiz and Question.

type Quiz struct {
	ID          uint   `json:"id"`
	UserID      string `json:"userId"`
	Author      string `json:"author"`
	Name        string `json:"name"`
	Description string `json:"description"`
	//has many relationship with Question
	Questions []Question `json:"questions" gorm:"constraint:OnDelete:CASCADE;"`
}
type Question struct {
	ID       uint   `json:"id"`
	Question string `json:"question"`
	//belongs to Quiz
	QuizID uint
}

I wanted to see what would happen if I sent a PUT request to replace the questions in a quiz with other questions as if I was using MongoDB and not MySQL.

quiz to update

{
        "id": 7,
        "userId": "642097c0f85c7e2a7c170460",
        "author": "User",
        "name": "Quiz 2",
        "description": "Description 2",
        "questions": [
            {
                "id": 5,
                "question": "How are you?",
                "QuizID": 7
            }
        ]
    }

PUT api/quiz/7 with empty array

{
    "name": "Quiz 2",
    "description": "Description 2",
    "questions": [
        
    ]
}

code

func UpdateQuiz(c *fiber.Ctx) error {
	// parse id
	id, err := c.ParamsInt("id")

	if err != nil {
		return c.Status(http.StatusBadRequest).JSON("Please make sure that :id is an integer")
	}

	// find quiz
	var quiz models.Quiz

	if err := findPreloadedQuiz(id, &quiz); err != nil {
		return c.Status(http.StatusBadRequest).JSON(err.Error())
	}

	fmt.Println(1, quiz)

	// check if author
	if ok := isAuthor(c, &quiz); !ok {
		return nil
	}

	//TODO how to update question

	if err := c.BodyParser(&quiz); err != nil {
		return c.Status(http.StatusInternalServerError).JSON(err.Error())
	}

	fmt.Println(2, quiz)

	// save
	database.Instance.Save(&quiz)
	return c.Status(http.StatusOK).JSON(quiz)
}

logs

[quiz-pod] 1 {7 642097c0f85c7e2a7c170460 User Quiz 2 Description 2 [{5 How are you? 7}]}
[quiz-pod] 2 {7 642097c0f85c7e2a7c170460 User Quiz 2 Description 2 []}
[quiz-pod]
[quiz-pod] 2023/03/26 20:04:25 /quiz/routes/quiz.go:123 Error 1062 (23000): Duplicate entry '7' for key 'PRIMARY'
[quiz-pod] [5.176ms] [rows:0] INSERT INTO `quizzes` (`user_id`,`author`,`name`,`description`,`id`) VALUES ('642097c0f85c7e2a7c170460','User','Quiz 2','Description 2',7)

I don't understand why the error Error 1062 (23000): Duplicate entry '7' for key 'PRIMARY' happens; is gorm trying to create a new quiz? (If I send a request without the questions field, the name and description do get updated)

The same error happens when trying to update a specific question.

{
    "name": "Quiz 2",
    "description": "Description 2",
    "questions": [
        {
            "id": 5,
            "question": "Why are you?",
            "QuizID": 7
        }
    ]
}

[quiz-pod] 1 {7 642097c0f85c7e2a7c170460 User Quiz 2 Description 2 [{5 How are you? 7}]}
[quiz-pod] 2 {7 642097c0f85c7e2a7c170460 User Quiz 2 Description 2 [{5 Why are you? 7}]}
[quiz-pod]
[quiz-pod] 2023/03/26 20:07:55 /quiz/routes/quiz.go:123 Error 1062 (23000): Duplicate entry '7' for key 'PRIMARY'
[quiz-pod] [1.970ms] [rows:0] INSERT INTO `quizzes` (`user_id`,`author`,`name`,`description`,`id`) VALUES ('642097c0f85c7e2a7c170460','User','Quiz 2','Description 2',7)

If the error was Duplicate entry '5' for key 'PRIMARY' instead, it could be interpreted as the question with id 5 already exists.

(Using database.Instance.Session(&gorm.Session{FullSaveAssociations: true}).Updates(&quiz) does work, but I still want to understand the error and why associations mode needs to be explicitly enabled. To be able to delete questions as well, I use database.Instance.Model(&quiz).Association("Questions").Replace(&quiz.Questions))

from what I got from your question, you'd like to achieve these two things:

1. Replace the questions of a specific quiz
2. Being able to edit a specific question by its id

If I understood well, the following code should manage all of them.

package main

import (
	"gorm.io/driver/mysql"
	"gorm.io/gorm"
)

// docker run --name some-mysql -e MYSQL_ROOT_PASSWORD=root -p 3306:3306 -d mysql:latest
type Quiz struct {
	ID uint `json:"id"`
	// UserID      string `json:"userId"`
	Author string `json:"author"`
	// Name        string `json:"name"`
	// Description string `json:"description"`
	// has many relationship with Question
	Questions []Question `json:"questions" gorm:"constraint:OnDelete:CASCADE;"`
}
type Question struct {
	ID       uint   `json:"id"`
	Question string `json:"question"`
	// belongs to Quiz
	QuizID uint
}

func main() {
	dsn := "root:root@tcp(127.0.0.1:3306)/questions?charset=utf8mb4&parseTime=True&loc=Local"
	db, err := gorm.Open(mysql.Open(dsn), &gorm.Config{})
	if err != nil {
		panic(err)
	}

	db.AutoMigrate(&Quiz{})
	db.AutoMigrate(&Question{})

	// seed data
	questionToIns := []Question{
		{ID: 1, Question: "question 1"},
		{ID: 2, Question: "question 2"},
	}

	quizToIns := []Quiz{
		{ID: 1, Author: "John Doe", Questions: questionToIns},
	}

	db.Create(quizToIns)

	// start replacing
	if err = db.Where("quiz_id = ?", 1).Delete(&Question{}).Error; err != nil {
		panic(err)
	}
	questionToUpd := []Question{
		{ID: 3, Question: "new question", QuizID: 1},
	}
	if err = db.Create(questionToUpd).Error; err != nil {
		panic(err)
	}
	// end replacing

	// start editing
	var question Question
	if err = db.Find(&question, 3).Error; err != nil {
		panic(err)
	}

	question.Question = "updated description"
	if err = db.Updates(question).Error; err != nil {
		panic(err)
	}
	// end editing
}

Let me share with you a couple of notes.
First, at the top of the file, you can see the command used to run a MySQL instance on my machine through Docker.
Then, you can see a code to seed sample data in the tables. Please be aware that I simplified a little your structs just for the sake of the demo.
Afterward, you can see the code dealing with the first requirement (questions replacement). This code is wrapped within the comment // start replacing and // end replacing.
Lastly, I added the code dealing with the editing requirement that is wrapped within the comments // start editing and // end editing.
That's it! You should be able to manage your requirements too, let me know!