Realm migrations in Swift

Question

I have a Realm Object modeled as so

class WorkoutSet: Object {
     // Schema 0
     dynamic var exerciseName: String = ""
     dynamic var reps: Int = 0
     // Schema 0 + 1
     dynamic var setCount: Int = 0
}

I am trying to perform a migration.

Within my AppDelegate I have imported RealmSwift.

Within the function didFinishLaunchWithOptions I call

Migrations().checkSchema()

Migrations is a class declared in another file.

Within that file there is a struct declared as so.

func checkSchema() {
    Realm.Configuration(
        // Set the new schema version. This must be greater than the previously used
        // version (if you've never set a schema version before, the version is 0).
        schemaVersion: 1,

        // Set the block which will be called automatically when opening a Realm with
        // a schema version lower than the one set above
        migrationBlock: { migration, oldSchemaVersion in
            // We haven’t migrated anything yet, so oldSchemaVersion == 0
            switch oldSchemaVersion {
            case 1:
                break
            default:
                // Nothing to do!
                // Realm will automatically detect new properties and removed properties
                // And will update the schema on disk automatically
                self.zeroToOne(migration)
            }
    })
}

func zeroToOne(migration: Migration) {
    migration.enumerate(WorkoutSet.className()) {
        oldObject, newObject in
        let setCount = 1
        newObject!["setCount"] = setCount
    }
}

I am getting an error for adding setCount to the model

What error do you get? Can you share that? – Shripada Jan 20 '16 at 05:00 — Shripada, Jan 20 '16 at 05:00
Property 'setCount' has been added to latest model – Cody Weaver Jan 20 '16 at 05:02 — Cody Weaver, Jan 20 '16 at 05:02

score 38 · Accepted Answer · edited Dec 11 '16 at 17:19

You will need to invoke the migration. Merely creating a configuration, won't invoke it. There are two ways of doing this:

Set your configuration with migration as Realm's default configuration-

let config = Realm.Configuration(
  // Set the new schema version. This must be greater than the previously used
  // version (if you've never set a schema version before, the version is 0).
  schemaVersion: 1,

  // Set the block which will be called automatically when opening a Realm with
  // a schema version lower than the one set above
  migrationBlock: { migration, oldSchemaVersion in

    if oldSchemaVersion < 1 {
      migration.enumerate(WorkoutSet.className()) { oldObject, newObject in
        newObject?["setCount"] = setCount
      }    
    }
  }
) 
Realm.Configuration.defaultConfiguration = config

OR

Migrate manually using migrateRealm :

migrateRealm(config)

Now your migration should work properly.

Is there a way to just do this automatically? I think this is pretty straight forward if the value could be nil... any thoughts? — Lucas Chwe, Nov 08 '18 at 19:56

score 6 · Answer 2 · answered Jan 20 '16 at 06:33

Because you just create Realm.Configuration. Migration block is invoked by Realm if needed. You cannot invoke migration directly.

So to be invoke migration block, you should set configuration object to Realm, or set as default configuration. Then create Realm instance.

So You need to do the following:

let config = Realm.Configuration(
    // Set the new schema version. This must be greater than the previously used
    // version (if you've never set a schema version before, the version is 0).
    schemaVersion: 1,

    // Set the block which will be called automatically when opening a Realm with
    // a schema version lower than the one set above
    migrationBlock: { migration, oldSchemaVersion in
        // We haven’t migrated anything yet, so oldSchemaVersion == 0
        switch oldSchemaVersion {
        case 1:
            break
        default:
            // Nothing to do!
            // Realm will automatically detect new properties and removed properties
            // And will update the schema on disk automatically
            self.zeroToOne(migration)
        }
})

let realm = try! Realm(configuration: config) // Invoke migration block if needed

Do not call `let realm = try! Realm(configuration: config)` with force. It's marked as throwing for a reason (for example there is no space on device to perform migration). Instead you should call it in a do-catch block. — Gienadij Mackiewicz, Jun 17 '19 at 08:43

score 0 · Answer 3 · answered Apr 06 '23 at 15:44

Update answer @kishikawa katsumi for Xcode 14.3:

func zeroToOne(migration: Migration) {
    migration.enumerateObjects(ofType: <Your Realm Object>.className()) { _, newObject in
        newObject![<Your new properties>] = nil
        // Ex: newObject!["release_date"] = nil
    }
}

let config = Realm.Configuration(
    // Set the new schema version. This must be greater than the previously used
    // version (if you've never set a schema version before, the version is 0).
    schemaVersion: 1,

    // Set the block which will be called automatically when opening a Realm with
    // a schema version lower than the one set above
    migrationBlock: { migration, oldSchemaVersion in

        // We haven’t migrated anything yet, so oldSchemaVersion == 0
        switch oldSchemaVersion {
        case 1:
            break
        default:
            // Nothing to do!
            // Realm will automatically detect new properties and removed properties
            // And will update the schema on disk automatically
            zeroToOne(migration: migration)
        }
    }
)

let realm = try! Realm(configuration: config) // Invoke migration block if needed

This doesn't update the referenced answer at all. You've copied that answer's code without any change. It seems you've only changed the original question's `zeroToOne` function. — HangarRash, Apr 06 '23 at 15:54
Yes. It changed from "enumerate" to "enumerateObjects(ofType:)" — kien-hoang, Apr 07 '23 at 16:02

Realm migrations in Swift

3 Answers3

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