Panduan Membuat Relasi pada ParseSwift iOS SDK di Back4App

16min
Hubungan
Pendahuluan
Hubungan adalah fitur dasar untuk mengorganisir objek data yang disimpan di database. ParseSwift menyediakan alat dan metode yang diperlukan untuk membangun hubungan antara kelas di Database Back4App Anda. Tergantung pada kasus penggunaan, kita dapat mengidentifikasi jenis hubungan berikut
1: Sebuah hubungan yang hanya menghubungkan dua objek data.
N: Sebuah hubungan antara satu objek data danN objek data
N:N: Sebuah hubungan antaraN objek data keN objek data.
Seperti yang akan kita lihat di bawah, menerapkan 1:1 hubungan relatif sederhana. Untuk 1:N dan N:N hubungan, implementasinya melibatkan objek ParseRelation yang disediakan oleh ParseSwift SDK. Ada alternatif tambahan untuk menerapkan 1:N dan N:N hubungan. Namun, karena efisiensi dan kinerja, kami merekomendasikan untuk mengikuti pendekatan kami
Tutorial ini menggunakan aplikasi dasar yang dibuat di Xcode 12 dan iOS 14.
Kapan saja, Anda dapat mengakses Proyek lengkap melalui repositori GitHub kami.
﻿Repositori Contoh iOS﻿
Tujuan
Untuk memahami bagaimana hubungan diterapkan dalam Database Back4App.
Prasyarat
Untuk menyelesaikan quickstart ini, Anda perlu:
Xcode.
Sebuah aplikasi yang dibuat di Back4App.
Ikuti tutorial Aplikasi Parse Baru untuk belajar bagaimana cara membuat aplikasi Parse di Back4App.
Catatan: Ikuti Tutorial Instalasi Parse SDK (Swift) untuk membuat Proyek Xcode yang terhubung ke Back4App.
Memahami Aplikasi Buku Kami
Template proyek adalah Aplikasi Buku di mana pengguna memasukkan detail buku untuk menyimpannya di Database Back4App. Di layar utama aplikasi, Anda akan menemukan formulir untuk melakukannya.
﻿
Menggunakan + tombol yang terletak di sisi kanan atas bilah navigasi, kita dapat menambahkan sebanyak mungkin Penerbit, Genre, dan Penulis sesuai kebutuhan. Setelah pengguna memasukkan sebuah buku, mereka dapat menggunakan tombol Tambah buku untuk menyimpan buku di Database Back4App mereka. Selain itu, tombol Daftar buku akan memungkinkan kita untuk menampilkan semua buku yang ditambahkan pengguna dan juga untuk melihat hubungan mereka dengan penerbit dan penulis.
Referensi cepat perintah yang akan kita gunakan
Kami menggunakan objek Penulis, Penerbit, ISBN dan Buku:
Genre
Author
Publisher
ISBN
Book
 import Foundation
 import ParseSwift
3
 struct Genre: ParseObject {
     ...
  
     var name: String?
  
     ...
}
 import Foundation
 import ParseSwift
3
 struct Genre: ParseObject {
     ...
  
     var name: String?
  
     ...
}
﻿
Sebelum menyimpan instance objek ini di Database Back4App, semua propertinya harus sesuai dengan Codable dan Hashable protokol.
Kami menggunakan metode berikut untuk mengelola objek-objek ini di Database Back4App:
Create
Read
Add relations
Query relations
Remaining relations
   //When creating a new instance of Author we use
 var newAuthor: Author = Author(name: "John Doe")
2
 // Saves newAuthor on your Back4App Database synchronously and returns the new saved Item. It throws and error if something went wrong.
 let savedAuthor = try? newAuthor.save()
5
 // Saves newAuthor on your Back4App Database asynchronously, and passes a Result<Author, ParseError> object to the completion block to handle the save process.
 newAuthor.save { result in
     // Handle the result to check wether the save process was successfull or not
 }
   //When creating a new instance of Author we use
 var newAuthor: Author = Author(name: "John Doe")
2
 // Saves newAuthor on your Back4App Database synchronously and returns the new saved Item. It throws and error if something went wrong.
 let savedAuthor = try? newAuthor.save()
5
 // Saves newAuthor on your Back4App Database asynchronously, and passes a Result<Author, ParseError> object to the completion block to handle the save process.
 newAuthor.save { result in
     // Handle the result to check wether the save process was successfull or not
 }
﻿
- Unduh Template Aplikasi Buku
Proyek XCode memiliki struktur berikut
﻿
Kapan saja, Anda dapat mengakses Proyek lengkap melalui repositori GitHub kami.
﻿Repositori Contoh iOS﻿
- Alur CRUD Tambahan

Sebelum melanjutkan, perlu untuk mengimplementasikan beberapa fungsi CRUD agar kita dapat menyimpan Penulis, Penerbit dan Genre objek. Di dalamMainController+ParseSwift.swift file, di bawah ekstensi untuk kelas MainController , kami mengimplementasikan metode berikut
Swift
 // MainController+ParseSwift.swift file
 extension MainController {
     /// Collects the data to save an instance of Book on your Back4App database.
     func saveBook() {
         view.endEditing(true)
      
         // 1. First retrieve all the information for the Book (title, isbn, etc)
         guard let bookTitle = bookTitleTextField.text else {
             return presentAlert(title: "Error", message: "Invalid book title")
        }
      
        guard let isbnValue = isbnTextField.text else {
            return presentAlert(title: "Error", message: "Invalid ISBN value.")
        }
      
        let query = ISBN.query("value" == isbnValue)
      
        guard (try? query.first()) == nil else {
            return presentAlert(title: "Error", message: "The entered ISBN already exists.")
        }
        
        guard let genreObjectId = genreOptionsView.selectedOptionIds.first,
              let genre = genres.first(where: { $0.objectId == genreObjectId})
        else {
            return presentAlert(title: "Error", message: "Invalid genre.")
        }
      
        guard let publishingYearString = publishingYearTextField.text, let publishingYear = Int(publishingYearString) else {
            return presentAlert(title: "Error", message: "Invalid publishing year.")
        }
        
        let authors: [Author] = self.authorOptionsView.selectedOptionIds.compactMap { [weak self] objectId in
           self?.authors.first(where: { objectId == $0.objectId })
        }
      
        let publishers: [Publisher] = self.publisherOptionsView.selectedOptionIds.compactMap { [weak self] objectId in
           self?.publishers.first(where: { objectId == $0.objectId })
        }
      
        // Since we are making multiple requests to Back4App, it is better to use synchronous methods and dispatch them on the background queue
        DispatchQueue.global(qos: .background).async {
            do {
                let isbn = ISBN(value: isbnValue) // 2. Instantiate a new ISBN object
              
                let savedBook = try Book( // 3. Instantiate a new Book object with the corresponding input fields
                    title: bookTitle,
                   publishingYear: publishingYear,
                    genre: genre,
                    isbn: isbn
                ).save() // 4. Save the new Book object
              
                ... // Here we will implement the relations
                
                DispatchQueue.main.async {
                    self.presentAlert(title: "Success", message: "Book saved successfully.")
                }
            } catch {
                DispatchQueue.main.async {
                   self.presentAlert(title: "Error", message: "Failed to save book: \((error as! ParseError).message)")
                }
            }
        }
    }
  
    /// Retrieves all the data saved under the Genre class in your Back4App Database
    func fetchGenres() {
        let query = Genre.query()
        
        query.find { [weak self] result in
            switch result {
            case .success(let genres):
                self?.genres = genres // When setting self?.genres, it triggers the corresponding UI update
            case .failure(let error):
                self?.presentAlert(title: "Error", message: error.message)
            }
        }
    }
  
    /// Presents a simple alert where the user can enter the name of a genre to save it on your Back4App Database
    func handleAddGenre() {
        // Displays a form with a single input and executes the completion block when the user presses the submit button
        presentForm(
            title: "Add genre",
            description: "Enter a description for the genre",
            placeholder: nil
        ) { [weak self] name in
            guard let name = name else { return }
            let genre = Genre(name: name)
            
            let query = Genre.query("name" == name)
          
            guard ((try? query.first()) == nil) else {
                self?.presentAlert(title: "Error", message: "This genre already exists.")
                return
            }
            
            genre.save { [weak self] result in
                switch result {
                case .success(let addedGenre):
                   self?.presentAlert(title: "Success", message: "Genre added!")
                   self?.genres.append(addedGenre)
               case .failure(let error):
                   self?.presentAlert(title: "Error", message: "Failed to save genre: \(error.message)")
               }
           }
       }
   }
   
   /// Retrieves all the data saved under the Publisher class in your Back4App Database
   func fetchPublishers() {
       let query = Publisher.query()
       
       query.find { [weak self] result in
           switch result {
           case .success(let publishers):
               self?.publishers = publishers
           case .failure(let error):
               self?.presentAlert(title: "Error", message: error.message)
           }
       }
   }
  
   /// Presents a simple alert where the user can enter the name of a publisher to save it on your Back4App Database
   func handleAddPublisher() {
       // Displays a form with a single input and executes the completion block when the user presses the submit button
       presentForm(
           title: "Add publisher",
           description: "Enter the name of the publisher",
           placeholder: nil
       ) { [weak self] name in
           guard let name = name else { return }
           
           let query = Publisher.query("name" == name)
           
           guard ((try? query.first()) == nil) else {
               self?.presentAlert(title: "Error", message: "This publisher already exists.")
               return
           }
           
           let publisher = Publisher(name: name)
           
           publisher.save { [weak self] result in
               switch result {
               case .success(let addedPublisher):
                   self?.presentAlert(title: "Success", message: "Publisher added!")
                   self?.publishers.append(addedPublisher)
               case .failure(let error):
                   self?.presentAlert(title: "Error", message: "Failed to save publisher: \(error.message)")
               }
           }
       }
   }
  
   /// Retrieves all the data saved under the Genre class in your Back4App Database
   func fetchAuthors() {
       let query = Author.query()
      
       query.find { [weak self] result in
           switch result {
           case .success(let authors):
               self?.authors = authors
           case .failure(let error):
               self?.presentAlert(title: "Error", message: error.message)
           }
       }
   }
  
   /// Presents a simple alert where the user can enter the name of an author to save it on your Back4App Database
   func handleAddAuthor() {
       // Displays a form with a single input and executes the completion block when the user presses the submit button
       presentForm(
           title: "Add author",
           description: "Enter the name of the author",
           placeholder: nil
       ) { [weak self] name in
           guard let name = name else { return }
          
           let query = Author.query("name" == name)
          
           guard ((try? query.first()) == nil) else {
               self?.presentAlert(title: "Error", message: "This author already exists.")
               return
           }
           
           let author = Author(name: name)
           
           author.save { [weak self] result in
               switch result {
               case .success(let addedAuthor):
                   self?.presentAlert(title: "Success", message: "Author added!")
                   self?.authors.append(addedAuthor)
               case .failure(let error):
                   self?.presentAlert(title: "Error", message: "Failed to save author: \(error.message)")
               }
           }
       }
   }
}sw
 // MainController+ParseSwift.swift file
 extension MainController {
     /// Collects the data to save an instance of Book on your Back4App database.
     func saveBook() {
         view.endEditing(true)
      
         // 1. First retrieve all the information for the Book (title, isbn, etc)
         guard let bookTitle = bookTitleTextField.text else {
             return presentAlert(title: "Error", message: "Invalid book title")
        }
      
        guard let isbnValue = isbnTextField.text else {
            return presentAlert(title: "Error", message: "Invalid ISBN value.")
        }
      
        let query = ISBN.query("value" == isbnValue)
      
        guard (try? query.first()) == nil else {
            return presentAlert(title: "Error", message: "The entered ISBN already exists.")
        }
        
        guard let genreObjectId = genreOptionsView.selectedOptionIds.first,
              let genre = genres.first(where: { $0.objectId == genreObjectId})
        else {
            return presentAlert(title: "Error", message: "Invalid genre.")
        }
      
        guard let publishingYearString = publishingYearTextField.text, let publishingYear = Int(publishingYearString) else {
            return presentAlert(title: "Error", message: "Invalid publishing year.")
        }
        
        let authors: [Author] = self.authorOptionsView.selectedOptionIds.compactMap { [weak self] objectId in
           self?.authors.first(where: { objectId == $0.objectId })
        }
      
        let publishers: [Publisher] = self.publisherOptionsView.selectedOptionIds.compactMap { [weak self] objectId in
           self?.publishers.first(where: { objectId == $0.objectId })
        }
      
        // Since we are making multiple requests to Back4App, it is better to use synchronous methods and dispatch them on the background queue
        DispatchQueue.global(qos: .background).async {
            do {
                let isbn = ISBN(value: isbnValue) // 2. Instantiate a new ISBN object
              
                let savedBook = try Book( // 3. Instantiate a new Book object with the corresponding input fields
                    title: bookTitle,
                   publishingYear: publishingYear,
                    genre: genre,
                    isbn: isbn
                ).save() // 4. Save the new Book object
              
                ... // Here we will implement the relations
                
                DispatchQueue.main.async {
                    self.presentAlert(title: "Success", message: "Book saved successfully.")
                }
            } catch {
                DispatchQueue.main.async {
                   self.presentAlert(title: "Error", message: "Failed to save book: \((error as! ParseError).message)")
                }
            }
        }
    }
  
    /// Retrieves all the data saved under the Genre class in your Back4App Database
    func fetchGenres() {
        let query = Genre.query()
        
        query.find { [weak self] result in
            switch result {
            case .success(let genres):
                self?.genres = genres // When setting self?.genres, it triggers the corresponding UI update
            case .failure(let error):
                self?.presentAlert(title: "Error", message: error.message)
            }
        }
    }
  
    /// Presents a simple alert where the user can enter the name of a genre to save it on your Back4App Database
    func handleAddGenre() {
        // Displays a form with a single input and executes the completion block when the user presses the submit button
        presentForm(
            title: "Add genre",
            description: "Enter a description for the genre",
            placeholder: nil
        ) { [weak self] name in
            guard let name = name else { return }
            let genre = Genre(name: name)
            
            let query = Genre.query("name" == name)
          
            guard ((try? query.first()) == nil) else {
                self?.presentAlert(title: "Error", message: "This genre already exists.")
                return
            }
            
            genre.save { [weak self] result in
                switch result {
                case .success(let addedGenre):
                   self?.presentAlert(title: "Success", message: "Genre added!")
                   self?.genres.append(addedGenre)
               case .failure(let error):
                   self?.presentAlert(title: "Error", message: "Failed to save genre: \(error.message)")
               }
           }
       }
   }
   
   /// Retrieves all the data saved under the Publisher class in your Back4App Database
   func fetchPublishers() {
       let query = Publisher.query()
       
       query.find { [weak self] result in
           switch result {
           case .success(let publishers):
               self?.publishers = publishers
           case .failure(let error):
               self?.presentAlert(title: "Error", message: error.message)
           }
       }
   }
  
   /// Presents a simple alert where the user can enter the name of a publisher to save it on your Back4App Database
   func handleAddPublisher() {
       // Displays a form with a single input and executes the completion block when the user presses the submit button
       presentForm(
           title: "Add publisher",
           description: "Enter the name of the publisher",
           placeholder: nil
       ) { [weak self] name in
           guard let name = name else { return }
           
           let query = Publisher.query("name" == name)
           
           guard ((try? query.first()) == nil) else {
               self?.presentAlert(title: "Error", message: "This publisher already exists.")
               return
           }
           
           let publisher = Publisher(name: name)
           
           publisher.save { [weak self] result in
               switch result {
               case .success(let addedPublisher):
                   self?.presentAlert(title: "Success", message: "Publisher added!")
                   self?.publishers.append(addedPublisher)
               case .failure(let error):
                   self?.presentAlert(title: "Error", message: "Failed to save publisher: \(error.message)")
               }
           }
       }
   }
  
   /// Retrieves all the data saved under the Genre class in your Back4App Database
   func fetchAuthors() {
       let query = Author.query()
      
       query.find { [weak self] result in
           switch result {
           case .success(let authors):
               self?.authors = authors
           case .failure(let error):
               self?.presentAlert(title: "Error", message: error.message)
           }
       }
   }
  
   /// Presents a simple alert where the user can enter the name of an author to save it on your Back4App Database
   func handleAddAuthor() {
       // Displays a form with a single input and executes the completion block when the user presses the submit button
       presentForm(
           title: "Add author",
           description: "Enter the name of the author",
           placeholder: nil
       ) { [weak self] name in
           guard let name = name else { return }
          
           let query = Author.query("name" == name)
          
           guard ((try? query.first()) == nil) else {
               self?.presentAlert(title: "Error", message: "This author already exists.")
               return
           }
           
           let author = Author(name: name)
           
           author.save { [weak self] result in
               switch result {
               case .success(let addedAuthor):
                   self?.presentAlert(title: "Success", message: "Author added!")
                   self?.authors.append(addedAuthor)
               case .failure(let error):
                   self?.presentAlert(title: "Error", message: "Failed to save author: \(error.message)")
               }
           }
       }
   }
}sw
﻿
Untuk detail lebih lanjut tentang langkah ini, Anda dapat pergi ke panduan operasi dasar.
- Menyiapkan hubungan
Sebelum mulai membuat hubungan, lihatlah referensi cepat bagian untuk mendapatkan gambaran tentang objek yang ingin kita hubungkan satu sama lain. Pada gambar di bawah ini, kami menunjukkan bagaimana objek-objek ini saling terkait
﻿
Seperti yang dapat dilihat, hubungan dibuat dengan menempatkan objek Buku di tengah. Panah menunjukkan bagaimana setiap objek terkait dengan objek Buku.
- Mengimplementasikan hubungan
Kasus 1:1
Menambahkan 1:1 hubungan dapat dengan mudah dicapai dengan menambahkan properti di objek Buku, yaitu,
Swift
 struct Book: ParseObject {
     ...
3
     var isbn: ISBN? // Esablishes a 1:1 relation between Book and ISBN
  
     ...
 }
 struct Book: ParseObject {
     ...
3
     var isbn: ISBN? // Esablishes a 1:1 relation between Book and ISBN
  
     ...
 }
﻿
Dalam hal ini, Buku dan ISBN memiliki hubungan 1:1 di mana Buku diidentifikasi sebagai orang tua dan ISBN sebagai anak. Secara internal, ketika Back4App menyimpan sebuah instance dari Buku, ia menyimpan objek ISBN (di bawah nama kelas ISBN ) terlebih dahulu. Setelah proses ini selesai, Back4App melanjutkan dengan objek Buku. Objek Buku baru disimpan sedemikian rupa sehingga properti isbn diwakili oleh objek Pointer<ISBN>. Sebuah Pointer<> memungkinkan kita untuk menyimpan sebuah instance unik dari objek ISBN yang terkait dengan orang tua yang sesuai.
Kasus 1:N
Untuk 1:N relasi, cara yang paling efisien untuk mengimplementasikannya adalah melalui objek ParseRelation<Book> . ParseSwift menyediakan serangkaian metode untuk menambahkan jenis hubungan ini untuk objek apa pun yang sesuai dengan ParseObject protokol. Misalnya, jika kita ingin membuat 1:N relasi antara Book dan Author kita dapat menggunakan
Swift
 let someBook: Book
 let authors: [Author]
 ...
4
 // We create a relation (identified by the name 'authors') between someBook and a set of authors
 let bookToAuthorsRelation = 
 guard let bookToAuthorsRelation = try someBook.relation?.add("authors", objects: authors) // Book -> Author 
 else {
     fatalError("Failed to add relation")
}
11
let savedRelation = try bookToAuthorsRelation.save() // Saves the relation synchronously
13
bookToAuthorsRelation.save { result in // Saves the relation asynchronously
    // Handle the result
}
 let someBook: Book
 let authors: [Author]
 ...
4
 // We create a relation (identified by the name 'authors') between someBook and a set of authors
 let bookToAuthorsRelation = 
 guard let bookToAuthorsRelation = try someBook.relation?.add("authors", objects: authors) // Book -> Author 
 else {
     fatalError("Failed to add relation")
}
11
let savedRelation = try bookToAuthorsRelation.save() // Saves the relation synchronously
13
bookToAuthorsRelation.save { result in // Saves the relation asynchronously
    // Handle the result
}
﻿
Sangat mudah untuk menyesuaikan cuplikan ini untuk hubungan lain yang kita miliki untuk Buku.
- Menggabungkan semuanya
Setelah kita menetapkan ide dasar untuk menerapkan hubungan, sekarang kita menyelesaikan saveBook() metode. Kita mencantumkan poin-poin kunci yang perlu diingat selama proses ini
Swift
 extension MainController {
     /// Collects the data to save an instance of Book on your Back4App database.
     func saveBook() {
         ...        
         // 1. First retrieve all the information for the Book (bookTitle, isbnValue, etc)
         ...
7
         // Since we are making multiple requests to Back4App, it is better to use synchronous methods and dispatch them on the background queue
         DispatchQueue.global(qos: .background).async {
            do {
                let isbn = ISBN(value: isbnValue) // 2. Instantiate a new ISBN object
              
                let savedBook = try Book( // 3. Instantiate a new Book object with the corresponding input fields
                    title: bookTitle,
                    publishingYear: publishingYear,
                    genre: genre,
                    isbn: isbn
                ).save() // 4. Save the new Book object
              
                // 5. Add the corresponding relations for new Book object
                guard let bookToAuthorsRelation = try savedBook.relation?.add("authors", objects: authors), // Book -> Author
                      let bootkToPublishersRelation = try savedBook.relation?.add("publishers", objects: publishers), // Book -> Publisher
                      let genreRelation = try genre.relation?.add("books", objects: [savedBook]) // Genre -> Book
                else {
                    return DispatchQueue.main.async {
                        self.presentAlert(title: "Error", message: "Failed to add relations")
                    }
                }
                               
                // 6. Save the relations
                _ = try bookToAuthorsRelation.save()
                _ = try bootkToPublishersRelation.save()
                _ = try genreRelation.save()
               
                DispatchQueue.main.async {
                    self.presentAlert(title: "Success", message: "Book saved successfully.")
                }
            } catch {
                DispatchQueue.main.async {
                    self.presentAlert(title: "Error", message: "Failed to save book: \((error as! ParseError).message)")
                }
            }
        }
    }
45
    ...
}
 extension MainController {
     /// Collects the data to save an instance of Book on your Back4App database.
     func saveBook() {
         ...        
         // 1. First retrieve all the information for the Book (bookTitle, isbnValue, etc)
         ...
7
         // Since we are making multiple requests to Back4App, it is better to use synchronous methods and dispatch them on the background queue
         DispatchQueue.global(qos: .background).async {
            do {
                let isbn = ISBN(value: isbnValue) // 2. Instantiate a new ISBN object
              
                let savedBook = try Book( // 3. Instantiate a new Book object with the corresponding input fields
                    title: bookTitle,
                    publishingYear: publishingYear,
                    genre: genre,
                    isbn: isbn
                ).save() // 4. Save the new Book object
              
                // 5. Add the corresponding relations for new Book object
                guard let bookToAuthorsRelation = try savedBook.relation?.add("authors", objects: authors), // Book -> Author
                      let bootkToPublishersRelation = try savedBook.relation?.add("publishers", objects: publishers), // Book -> Publisher
                      let genreRelation = try genre.relation?.add("books", objects: [savedBook]) // Genre -> Book
                else {
                    return DispatchQueue.main.async {
                        self.presentAlert(title: "Error", message: "Failed to add relations")
                    }
                }
                               
                // 6. Save the relations
                _ = try bookToAuthorsRelation.save()
                _ = try bootkToPublishersRelation.save()
                _ = try genreRelation.save()
               
                DispatchQueue.main.async {
                    self.presentAlert(title: "Success", message: "Book saved successfully.")
                }
            } catch {
                DispatchQueue.main.async {
                    self.presentAlert(title: "Error", message: "Failed to save book: \((error as! ParseError).message)")
                }
            }
        }
    }
45
    ...
}
﻿
- Menanyakan relasi
Untuk 1:1 relasi, mengingat induk Book dan anaknya ISBN, kita menanyakan anak yang sesuai dengan menyertakannya dalam Book query
Swift
 let query = Book.query().include("isbn")
2
 let books = try query.find() // Retrieves synchronously all the books together with its isbn
4
 query.find { result in // Retrieves asynchronously all the books together with its isbn
     // Handle the result
 }
8
 let query = Book.query().include("isbn")
2
 let books = try query.find() // Retrieves synchronously all the books together with its isbn
4
 query.find { result in // Retrieves asynchronously all the books together with its isbn
     // Handle the result
 }
8
﻿
Dengan ini, semua buku dari query juga akan memiliki properti isbn yang diatur dengan benar dengan objek ISBN terkait.
Di sisi lain, untuk mengambil objek dengan hubungan 1:N , protokol ParseObject menyediakan metode statis queryRelation(_,parent:). Dengan memberikan nama hubungan (sebagai parameter pertama) dan orang tua, metode ini memungkinkan kita untuk membuat kueri yang diperlukan. Misalnya, untuk mengambil semua Author yang terkait dengan Book tertentu, kita dapat menggunakan cuplikan berikut
Swift
 let book: Book // Book from which we are trying to retrieve its related authors
2
 do {
     let authorsQuery = try Author.queryRelations("authors", parent: book) // 'authors' is the name of the relation it was saved with
      
     authorsQuery.find { [weak self] result in
         switch result {
         case .success(let authors):
             self?.authors = authors
                  
            DispatchQueue.main.async {
                // Update the UI
            }
            case .failure(let error):
                DispatchQueue.main.async {
                    self?.presentAlert(title: "Error", message: "Failed to retrieve authors: \(error.message)")
                }
            }
        }
    }
} catch {
    if let error = error as? ParseError {
        presentAlert(title: "Error", message: "Failed to retrieve authors: \(error.message)")
    } else {
        presentAlert(title: "Error", message: "Failed to retrieve authors: \(error.localizedDescription)")
    }
}
 let book: Book // Book from which we are trying to retrieve its related authors
2
 do {
     let authorsQuery = try Author.queryRelations("authors", parent: book) // 'authors' is the name of the relation it was saved with
      
     authorsQuery.find { [weak self] result in
         switch result {
         case .success(let authors):
             self?.authors = authors
                  
            DispatchQueue.main.async {
                // Update the UI
            }
            case .failure(let error):
                DispatchQueue.main.async {
                    self?.presentAlert(title: "Error", message: "Failed to retrieve authors: \(error.message)")
                }
            }
        }
    }
} catch {
    if let error = error as? ParseError {
        presentAlert(title: "Error", message: "Failed to retrieve authors: \(error.message)")
    } else {
        presentAlert(title: "Error", message: "Failed to retrieve authors: \(error.localizedDescription)")
    }
}
﻿
Demikian pula, kita dapat menanyakan objek terkait lainnya seperti Publisher.
Dalam file BookDetailsController.swift kita mengimplementasikan kueri ini untuk menampilkan hubungan yang dimiliki sebuah buku dengan penulis dan penerbit
Swift
// BookDetailsController.swift file
class BookDetailsController: UITableViewController {
    ...
  
    /// Retrieves the book's details, i.e., its relation with authors and publishers
    private func fetchDetails() {
        do {
            // Constructs the relations you want to query
            let publishersQuery = try Publisher.queryRelations("publishers", parent: book)
            let authorsQuery = try Author.queryRelations("authors", parent: book)
          
            // Obtains the publishers related to book and display them on the tableView, it presents an error if happened.
            publishersQuery.find { [weak self] result in
                switch result {
                case .success(let publishers):
                    self?.publishers = publishers
                  
                    // Update the UI
                    DispatchQueue.main.async {
                        self?.tableView.reloadSections(IndexSet([Section.publisher.rawValue]), with: .none)
                    }
                case .failure(let error):
                    DispatchQueue.main.async {
                        self?.presentAlert(title: "Error", message: "Failed to retrieve publishers: \(error.message)")
                    }
                }
            }
          
            // Obtains the authors related to book and display them on the tableView, it presents an error if happened.
            authorsQuery.find { [weak self] result in
                switch result {
                case .success(let authors):
                    self?.authors = authors
                  
                    // Update the UI
                    DispatchQueue.main.async {
                        self?.tableView.reloadSections(IndexSet([Section.author.rawValue]), with: .none)
                    }
                case .failure(let error):
                    DispatchQueue.main.async {
                        self?.presentAlert(title: "Error", message: "Failed to retrieve authors: \(error.message)")
                    }
                }
            }
        } catch { // If there was an error during the creation of the queries, this block should catch it
            if let error = error as? ParseError {
                presentAlert(title: "Error", message: "Failed to retrieve authors: \(error.message)")
            } else {
                presentAlert(title: "Error", message: "Failed to retrieve authors: \(error.localizedDescription)")
            }
        }
    }
  
    ...
}
// BookDetailsController.swift file
class BookDetailsController: UITableViewController {
    ...
  
    /// Retrieves the book's details, i.e., its relation with authors and publishers
    private func fetchDetails() {
        do {
            // Constructs the relations you want to query
            let publishersQuery = try Publisher.queryRelations("publishers", parent: book)
            let authorsQuery = try Author.queryRelations("authors", parent: book)
          
            // Obtains the publishers related to book and display them on the tableView, it presents an error if happened.
            publishersQuery.find { [weak self] result in
                switch result {
                case .success(let publishers):
                    self?.publishers = publishers
                  
                    // Update the UI
                    DispatchQueue.main.async {
                        self?.tableView.reloadSections(IndexSet([Section.publisher.rawValue]), with: .none)
                    }
                case .failure(let error):
                    DispatchQueue.main.async {
                        self?.presentAlert(title: "Error", message: "Failed to retrieve publishers: \(error.message)")
                    }
                }
            }
          
            // Obtains the authors related to book and display them on the tableView, it presents an error if happened.
            authorsQuery.find { [weak self] result in
                switch result {
                case .success(let authors):
                    self?.authors = authors
                  
                    // Update the UI
                    DispatchQueue.main.async {
                        self?.tableView.reloadSections(IndexSet([Section.author.rawValue]), with: .none)
                    }
                case .failure(let error):
                    DispatchQueue.main.async {
                        self?.presentAlert(title: "Error", message: "Failed to retrieve authors: \(error.message)")
                    }
                }
            }
        } catch { // If there was an error during the creation of the queries, this block should catch it
            if let error = error as? ParseError {
                presentAlert(title: "Error", message: "Failed to retrieve authors: \(error.message)")
            } else {
                presentAlert(title: "Error", message: "Failed to retrieve authors: \(error.localizedDescription)")
            }
        }
    }
  
    ...
}
﻿
- Jalankan aplikasi!
Sebelum menekan tombol jalankan di XCode, jangan lupa untuk mengonfigurasi Back4App  aplikasi Anda di AppDelegate kelas!
Anda harus menambahkan beberapa Genre’s, Publisher’s dan Author’s sebelum menambahkan buku baru. Kemudian, Anda dapat mulai memasukkan informasi buku untuk menyimpannya di Database Back4App Anda. Setelah Anda menyimpan satu Buku, buka dasbor Back4App dan pergi ke aplikasi Anda yang terhubung dengan proyek XCode. Di bagian Database, Anda akan menemukan kelas Book di mana semua buku yang dibuat oleh Aplikasi iOS disimpan.
Selain itu, Anda dapat melihat bahwa Back4App secara otomatis membuat kelas ISBN untuk menghubungkannya dengan objek Book. Jika Anda kembali ke kelas Book, Anda dapat mengidentifikasi tipe data untuk setiap jenis hubungan. Dalam kasus ISBN dan Genre, tipe data adalah Pointer<>. Di sisi lain, untuk hubungan seperti Author dan Publisher, tipe data adalah Relation<>. Ini adalah perbedaan kunci yang perlu diingat saat membangun hubungan.