About transaction in Realm database - ios

Question

This is my Realm object:
class AchievementDate : Object {
dynamic var date: Date = Date()
dynamic var apple: Int = Int(0)
func save() {
do {
let realm = try Realm()
try realm.write {
realm.add(self)
}
} catch let error as NSError {
fatalError(error.localizedDescription)
}
}
}
I change apple's value in View controller's viewDidLoad() method, as you can see:
override func viewDidLoad() {
super.viewDidLoad()
achievementDate.apple = 2
achievementDate.save()
}
then I update the apple's value when user clicks the pause button on the screen like this:
#IBAction func pausedButtonTapped(_ sender: UIButton) {
achievementDate.apple += 1
achievementDate.save()
}
Xcode runs it successfully but when I click the pause button, the app crashes. In the console it says:
*** Terminating app due to uncaught exception 'RLMException', reason: 'Attempting to modify object outside of a write transaction - call
beginWriteTransaction on an RLMRealm instance first.'
I am quite confused about this, btw, what does transaction mean in general? Thanks a lot.

Answer 1

A write transaction is used to group modifications of objects within a Realm into a single unit of work. Managed Realm objects may only be modified within a write transaction. The write transaction is scoped to the block you pass to the call to Realm.write(_:). The call to write begins a write transaction, the body is executed with the transaction active, and when the block returns the write transaction is committed and the changes are persisted to the Realm file.
You've not shared how achievementDate is initialized, but it seems safe to assume based on the exception you're seeing that it is an AchievementDate instance that is a managed object (that is, it was either created then added to a Realm, or it was loaded from a Realm). As the exception notes, you can only modify managed objects within a write transaction. You can either expand the scope of your write transaction to encompass the modification of the managed object, or you can avoid modifying a managed object altogether (by adding a primary key to your model class, and using either Realm.create(_:value:update:) or Realm.add(_:update:) with update: true to update an existing object with the given primary key value).

Answer 2

First, you need to define a Primary Key if you want to update the model:
class AchievementDate: Object {
dynamic var id = 0
dynamic var date:Date = Date()
dynamic var apple:Int = Int(0)
override static func primaryKey() -> String? {
return "id"
}
}
Then update the model like:
static func save(achievementDate:AchievementDate) {
let realm = try! Realm()
try! realm.write {
realm.add(achievementDate, update: true)
}
}
Alternatively, if you don't want primary Key, and the model is already retrieved from Realm, you could update the model like this:
let realm = try! Realm()
try! realm.write {
achievementDate.apple = 2
}