การเล่น Relation กับ GORM

package main

import (
  "gorm.io/gorm"
)

type Book struct {
  gorm.Model
  Name        string `json:"name"`
  Author      string `json:"author"`
  Description string `json:"description"`
  PublisherID uint
  Publisher   Publisher
  Authors     []Author `gorm:"many2many:author_books;"`
}

type Publisher struct {
  gorm.Model
  Details string
  Name    string
}

type Author struct {
  gorm.Model
  Name  string
  Books []Book `gorm:"many2many:author_books;"`
}

type AuthorBook struct {
  AuthorID uint
  Author   Author
  BookID   uint
  Book     Book
}

func createPublisher(db *gorm.DB, publisher *Publisher) error {
  result := db.Create(publisher)
  if result.Error != nil {
    return result.Error
  }
  return nil
}

func createAuthor(db *gorm.DB, author *Author) error {
  result := db.Create(author)
  if result.Error != nil {
    return result.Error
  }
  return nil
}

func createBookWithAuthor(db *gorm.DB, book *Book, authorIDs []uint) error {
  // First, create the book
  if err := db.Create(book).Error; err != nil {
    return err
  }

  return nil
}

func getBookWithPublisher(db *gorm.DB, bookID uint) (*Book, error) {
  var book Book
  result := db.Preload("Publisher").First(&book, bookID)
  if result.Error != nil {
    return nil, result.Error
  }
  return &book, nil
}

func getBookWithAuthors(db *gorm.DB, bookID uint) (*Book, error) {
  var book Book
  result := db.Preload("Authors").First(&book, bookID)
  if result.Error != nil {
    return nil, result.Error
  }
  return &book, nil
}

func listBooksOfAuthor(db *gorm.DB, authorID uint) ([]Book, error) {
  var books []Book
  result := db.Joins("JOIN author_books on author_books.book_id = books.id").
    Where("author_books.author_id = ?", authorID).
    Find(&books)
  if result.Error != nil {
    return nil, result.Error
  }
  return books, nil
}

func main() {
  // Migrate the schema
  db.AutoMigrate(&Book{}, &Publisher{}, &Author{}, &AuthorBook{})

  // ขาสร้าง
  publisher := Publisher{
    Details: "Publisher Details",
    Name:    "Publisher Name",
  }
  _ = createPublisher(db, &publisher)

  // Example data for a new author
  author := Author{
    Name: "Author Name",
  }
  _ = createAuthor(db, &author)

  // // Example data for a new book with an author
  book := Book{
    Name:        "Book Title",
    Author:      "Book Author",
    Description: "Book Description",
    PublisherID: publisher.ID,     // Use the ID of the publisher created above
    Authors:     []Author{author}, // Add the created author
  }
  _ = createBookWithAuthor(db, &book, []uint{author.ID})

  // ขาเรียก

  // Example: Get a book with its publisher
  bookWithPublisher, err := getBookWithPublisher(db, 1) // assuming a book with ID 1
  if err != nil {
      // Handle error
  }

  // Example: Get a book with its authors
  bookWithAuthors, err := getBookWithAuthors(db, 1) // assuming a book with ID 1
  if err != nil {
      // Handle error
  }

  // Example: List books of a specific author
  authorBooks, err := listBooksOfAuthor(db, 1) // assuming an author with ID 1
  if err != nil {
      // Handle error
  }

  fmt.Println(bookWithPublisher)
  fmt.Println(bookWithAuthors)
  fmt.Println(authorBooks)
}