Saving Coredata To-Many Relationships in Swift

Saving CoreData to-many relationships in Swift

In a one-to-many relationship, it is easier to set the "to-one" direction of the
inverse relationships, in your case just

list.board = board

so that the extension methods are actually not needed here.

swift core data how should I save and fetch the one to many relationship and assign value to my viewModel entity

Rather creating relation like this you can do it saving the class id in teacher and student and get them on the basis of id like as:

class ClassEntity {
    let id:String
    var classTitleName: String
    var classRank: Int

    
    init (id:String,
          classTitleName: String,
          classRank: Int) {
        self.id = id
        self.classTitleName = classTitleName
        self.classRank = classRank
    }
    
}

then your teacher and student

class StudentEntity {
    var classId:String
    var studentName: String
    var studentGender: String
    
    init(classId:String,studentName: String,
         studentGender: String){
        self.classId = classId
        self.studentName = studentName
        self.studentGender = studentGender
    }
}

class TeacherEntity {
    var classId:String
    var teacherName: String
    var teacherGender: String
    
    init(classId:String,teacherName: String,
         teacherGender: String){
        self.classId = classId
        self.teacherName = teacherName
        self.teacherGender = teacherGender
    }
}

then create coredatastack

import Foundation
import CoreData

class CoreDataStack: NSObject {
   
    private let modelName: String
    lazy var managedContext: NSManagedObjectContext = {
        return self.storeContainer.viewContext
    }()
    
    init(modelName: String) {
        self.modelName = modelName
    }
    
    private lazy var storeContainer: NSPersistentContainer = {
       
        let container = NSPersistentContainer(name: self.modelName)
        container.loadPersistentStores { (storeDescription, error) in
            if let error = error as NSError? {
                print("Unresolved error \(error), \(error.userInfo)")
            }
        }
        container.viewContext.automaticallyMergesChangesFromParent = true
        container.viewContext.mergePolicy = NSMergeByPropertyStoreTrumpMergePolicy
        return container
    }()
    
    
    func saveContext () {
        guard managedContext.hasChanges else { return }
        do {
            try managedContext.save()
        } catch let error as NSError {
            print("Unresolved error \(error), \(error.userInfo)")
        }
    }
    
    func updateContext() {
        do {
            try managedContext.save()
        } catch let error as NSError {
            print("Unresolved error \(error), \(error.userInfo)")
        }
    }
    
    func clearChanges() {
        managedContext.rollback()
    }
    
    func deleteReminder(reminder:Reminder) {
        managedContext.delete(reminder)
        updateContext()
    }
    
    func deleteNote(note:Note) {
        managedContext.delete(note)
        updateContext()
    }
    
    func deleteFromIds(selectedId:SubCategoryIds) {
        managedContext.delete(selectedId)
        updateContext()
    }
    
}

then create a manager

import Foundation
import CoreData

class CoreDataManager {
    
    static let shared = CoreDataManager()
    private init(){}
    lazy var coreDataStack = CoreDataStack(modelName: "YourModelName")
    
    func allClasses() -> [ClassEntity] {
        let fetechRequest: NSFetchRequest<ClassEntity> = ClassEntity.fetchRequest()
        do {
            let results = try coreDataStack.managedContext.fetch(fetechRequest)
            return results
        } catch {
            
        }
        return [ClassEntity]()
    }
    
    func allStudentOfClass(classId:String) -> [StudentEntity] {
        let fetechRequest: NSFetchRequest< StudentEntity > = StudentEntity.fetchRequest()
        fetechRequest.predicate = NSPredicate(format: "classId == %@",
                                              argumentArray: [classId])
        do {
            let results = try coreDataStack.managedContext.fetch(fetechRequest)
            return results
        } catch {
            
        }
        return [StudentEntity]()
    }
    
      func allTeacherOfClass(classId:String) -> [TeacherEntity] {
         let fetechRequest: NSFetchRequest<TeacherEntity> = TeacherEntity.fetchRequest()
       fetechRequest.predicate = NSPredicate(format: "classId == %@",
                                          argumentArray: [classId])
        do {
          let results = try coreDataStack.managedContext.fetch(fetechRequest)
          return results
       } catch {

    }
    return [TeacherEntity]()
  }
  
    
}
    
   

how you can create a class

 func createClass() {
    let id = "Class 1" //or whatever you want to say here
    let _class = Class(context: CoreDataManger.shared.coreDataStack.managedContext)
    _class.id = id
    _class.classTitleName = "Title"
    _class.classRank = 12
    CoreDataManger.shared.coreDataStack.saveContext()
}

same as how create teacher with this class

 func addTeacher(classId:String) {
        let _class = TeacherEntity(context: CoreDataManger.shared.coreDataStack.managedContext)
        _class.classId = classId
        _class.teacherGender = "Male"
        _class.teacherName = "Alex"
        CoreDataManger.shared.coreDataStack.saveContext()
    }

Swift CoreData, saving data via a many to many relationship

You need to create the parent or assign an existing object as the parent, it doesn't just exist automatically.

CoreData • How to modify a many-to-many-Relationship in CoreData using Swift 5?

When you create relationships in your model then Xcode will generate methods for getting and setting data through does relationships. For a to-many relationship you will get a getter <relationship name> and two setters addTo<relationship name>

So in this case you can update the relationships using addToBooks and addToPersons

Many To Many with additional data using Swift and Core Data

Core Data hides the correlation table if you use Many-2-Many (without any additional attributes). This means, in the inverse way, that you need to create a correlation table yourself. You can do this, if you create a new Entity in Core Data, which should have a 2-One relationship to both other Entities.

Without extra attributes:

Entity1 <-> Entity2 (The correlation table is hidden, but is there)

With extra attributes:

Entity1 <- CorrelationEntity(with extra attributes) -> Entity2

Core Data One to Many Relationship not saving correctly/as expected

You are creating a new, single instance of newFaction outside of your for faction in arrayOfFactions loop. So that single newFaction is used each time the loop runs and just assigns new values to it (overwriting previous values) resulting in it ending up with the last set of values. Hence you seeing a single faction with the 'last' set of values. Move the line:
Let newFaction = Factions(context: contectTCCEDJ)
Inside (at the beginning) of the for in loop.

Edit: you are adding your faction to a Set (which by definition requires unique entities) so all you're currently doing is re-adding the same faction each time rather than a new one. To-many relationships point to a Set.

Saving data into a One-to-many CoreData relationship in Swift

Comment has a to-one relationship to Listing. Just set this relationship.

newComment.listing = listing

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