Change an Object Model - Swift SDK

Overview

When you update your object schema, you must increment the schema version and perform a migration.

Seealso: This page provides general Swift and Objective-C migration examples. If you are using Realm with SwiftUI, see the SwiftUI-specific migration examples.

If your schema update adds optional properties or removes properties, Realm can perform the migration automatically. You only need to increment the schemaVersion.

For more complex schema updates, you must also manually specify the migration logic in a migrationBlock. This might include changes such as:

Adding required properties that must be populated with default values
Combining fields
Renaming a field
Changing a field's type
Converting from an object to an embedded object

Tip: When developing or debugging your application, you may prefer to delete the realm instead of migrating it. Use the deleteRealmIfMigrationNeeded flag to delete the database automatically when a schema mismatch would require a migration.

Never release an app to production with this flag set to true.

Schema Version

A schema version identifies the state of a Realm Schema at some point in time. Realm tracks the schema version of each realm and uses it to map the objects in each realm to the correct schema.

Schema versions are integers that you may include in the realm configuration when you open a realm. If a client application does not specify a version number when it opens a realm then the realm defaults to version 0.

Important: Migrations must update a realm to a higher schema version. Realm will throw an error if a client application opens a realm with a schema version that is lower than the realm's current version or if the specified schema version is the same as the realm's current version but includes different object schemas.

Migrations

Local migrations have access to the existing Realm Schema, version, and objects and define logic that incrementally updates the realm to its new schema version. To perform a local migration you must specify a new schema version that is higher than the current version and provide a migration function when you open the out-of-date realm.

In iOS, you can update underlying data to reflect schema changes using manual migrations. During such a manual migration, you can define new and deleted properties when they are added or removed from your schema.

Automatically Update Schema

Add a Property

Realm can automatically migrate added properties, but you must specify an updated schema version when you make these changes.

Note: Realm does not automatically set values for new required properties. You must use a migration block to set default values for new required properties. For new optional properties, existing records can have null values. This means you don't need a migration block when adding optional properties.

Example: A realm using schema version 1 has a Person object type that has first name, last name, and age properties:

Objective-C

objectivec

// In the first version of the app, the Person model
// has separate fields for first and last names,
// and an age property.
@interface Person : RLMObject
@property NSString *firstName;
@property NSString *lastName;
@property int age;
@end

@implementation Person
+ (NSArray<NSString *> *)requiredProperties {
    return @[@"firstName", @"lastName", @"age"];
}
@end

Swift

swift

// In the first version of the app, the Person model
// has separate fields for first and last names,
// and an age property.
class Person: Object {
    @Persisted var firstName = ""
    @Persisted var lastName = ""
    @Persisted var age = 0
}

The developer decides that the Person class needs an email field and updates the schema.

Objective-C

objectivec

// In a new version, you add a property
// on the Person model.
@interface Person : RLMObject
@property NSString *firstName;
@property NSString *lastName;
// Add a new "email" property.
@property NSString *email;
// New properties can be migrated
// automatically, but must update the schema version.
@property int age;
@end

@implementation Person
+ (NSArray<NSString *> *)requiredProperties {
    return @[@"firstName", @"lastName", @"email", @"age"];
}
@end

Swift

swift

// In a new version, you add a property
// on the Person model.
class Person: Object {
    @Persisted var firstName = ""
    @Persisted var lastName = ""
    // Add a new "email" property.
    @Persisted var email: String?
    // New properties can be migrated
    // automatically, but must update the schema version.
    @Persisted var age = 0

}

Realm automatically migrates the realm to conform to the updated Person schema. But the developer must set the realm's schema version to 2.

Objective-C

objectivec

// When you open the realm, specify that the schema
// is now using a newer version.
RLMRealmConfiguration *config = [[RLMRealmConfiguration alloc] init];
// Set the new schema version
config.schemaVersion = 2;
// Use this configuration when opening realms
[RLMRealmConfiguration setDefaultConfiguration:config];
RLMRealm *realm = [RLMRealm defaultRealm];

Swift

swift

// When you open the realm, specify that the schema
// is now using a newer version.
let config = Realm.Configuration(
    schemaVersion: 2)
// Use this configuration when opening realms
Realm.Configuration.defaultConfiguration = config
let realm = try! Realm()

Delete a Property

To delete a property from a schema, remove the property from the object's class and set a schemaVersion of the realm's configuration object. Deleting a property will not impact existing objects.

Example: A realm using schema version 1 has a Person object type that has first name, last name, and age properties:

Objective-C

objectivec

// In the first version of the app, the Person model
// has separate fields for first and last names,
// and an age property.
@interface Person : RLMObject
@property NSString *firstName;
@property NSString *lastName;
@property int age;
@end

@implementation Person
+ (NSArray<NSString *> *)requiredProperties {
    return @[@"firstName", @"lastName", @"age"];
}
@end

Swift

swift

// In the first version of the app, the Person model
// has separate fields for first and last names,
// and an age property.
class Person: Object {
    @Persisted var firstName = ""
    @Persisted var lastName = ""
    @Persisted var age = 0
}

The developer decides that the Person does not need the age field and updates the schema.

Objective-C

objectivec

// In a new version, you remove a property
// on the Person model.
@interface Person : RLMObject
@property NSString *firstName;
@property NSString *lastName;
// Remove the "age" property.
// @property int age;
// Removed properties can be migrated
// automatically, but must update the schema version.
@end

@implementation Person
+ (NSArray<NSString *> *)requiredProperties {
    return @[@"firstName", @"lastName"];
}
@end

Swift

swift

// In a new version, you remove a property
// on the Person model.
class Person: Object {
    @Persisted var firstName = ""
    @Persisted var lastName = ""
    // Remove the "age" property.
    // @Persisted var age = 0
    // Removed properties can be migrated
    // automatically, but must update the schema version.

}

Realm automatically migrates the realm to conform to the updated Person schema. But the developer must set the realm's schema version to 2.

Objective-C

objectivec

// When you open the realm, specify that the schema
// is now using a newer version.
RLMRealmConfiguration *config = [[RLMRealmConfiguration alloc] init];
// Set the new schema version
config.schemaVersion = 2;
// Use this configuration when opening realms
[RLMRealmConfiguration setDefaultConfiguration:config];
RLMRealm *realm = [RLMRealm defaultRealm];

Swift

swift

// When you open the realm, specify that the schema
// is now using a newer version.
let config = Realm.Configuration(
    schemaVersion: 2)
// Use this configuration when opening realms
Realm.Configuration.defaultConfiguration = config
let realm = try! Realm()

Tip: SwiftUI developers may see an error that a migration is required when they add or delete properties. This is related to the lifecycle in SwiftUI. The Views are laid out, and then the .environment modifier sets the config.

To resolve a migration error in these circumstances, pass Realm.Configuration(schemaVersion: <Your Incremented Version>) into the ObservedResults constructor.

Manually Migrate Schema

For more complex schema updates, Realm requires a manual migration for old instances of a given object to the new schema.

Rename a Property

To rename a property during a migration, use the Migration.renameProperty(onType:from:to:) method.

Realm applies any new nullability or indexing settings during the rename operation.

Example: Rename age to yearsSinceBirth within a migrationBlock.

Objective-C

objectivec

RLMRealmConfiguration *config = [[RLMRealmConfiguration alloc] init];
config.schemaVersion = 2;
config.migrationBlock = ^(RLMMigration * _Nonnull migration, uint64_t oldSchemaVersion) {
    if (oldSchemaVersion < 2) {
        // Rename the "age" property to "yearsSinceBirth".
        // The renaming operation should be done outside of calls to `enumerateObjects(ofType: _:)`.
        [migration renamePropertyForClass:[Person className] oldName:@"age" newName:@"yearsSinceBirth"];
    }
};

Swift

swift

let config = Realm.Configuration(
    schemaVersion: 2,
    migrationBlock: { migration, oldSchemaVersion in
        if oldSchemaVersion < 2 {
            // Rename the "age" property to "yearsSinceBirth".
            // The renaming operation should be done outside of calls to `enumerateObjects(ofType: _:)`.
            migration.renameProperty(onType: Person.className(), from: "age", to: "yearsSinceBirth")
        }
    })

Modify Properties

Tip: You can use the deleteRealmIfMigrationNeeded method to delete the realm if it would require a migration. This can be useful during development when you need to iterate quickly and don't want to perform the migration.

To define custom migration logic, set the migrationBlock property of the Configuration when opening a realm.

The migration block receives a Migration object that you can use to perform the migration. You can use the Migration object's enumerateObjects(ofType:_:) method to iterate over and update all instances of a given Realm type in the realm.

Example: A realm using schema version 1 has a Person object type that has separate fields for first and last names:

Objective-C

objectivec

// In the first version of the app, the Person model
// has separate fields for first and last names,
// and an age property.
@interface Person : RLMObject
@property NSString *firstName;
@property NSString *lastName;
@property int age;
@end

@implementation Person
+ (NSArray<NSString *> *)requiredProperties {
    return @[@"firstName", @"lastName", @"age"];
}
@end

Swift

swift

// In the first version of the app, the Person model
// has separate fields for first and last names,
// and an age property.
class Person: Object {
    @Persisted var firstName = ""
    @Persisted var lastName = ""
    @Persisted var age = 0
}

The developer decides that the Person class should use a combined fullName field instead of the separate firstName and lastName fields and updates the schema.

To migrate the realm to conform to the updated Person schema, the developer sets the realm's schema version to 2 and defines a migration function to set the value of fullName based on the existing firstName and lastName properties.

Objective-C

objectivec

// In version 2, the Person model has one
// combined field for the full name and age as a Int.
// A manual migration will be required to convert from
// version 1 to this version.
@interface Person : RLMObject
@property NSString *fullName;
@property int age;
@end

@implementation Person
+ (NSArray<NSString *> *)requiredProperties {
    return @[@"fullName", @"age"];
}
@end

objectivec

RLMRealmConfiguration *config = [[RLMRealmConfiguration alloc] init];
// Set the new schema version
config.schemaVersion = 2;
config.migrationBlock = ^(RLMMigration * _Nonnull migration, uint64_t oldSchemaVersion) {
    if (oldSchemaVersion < 2) {
        // Iterate over every 'Person' object stored in the Realm file to
        // apply the migration
        [migration enumerateObjects:[Person className]
                            block:^(RLMObject * _Nullable oldObject, RLMObject * _Nullable newObject) {
            // Combine name fields into a single field
            newObject[@"fullName"] = [NSString stringWithFormat:@"%@ %@",
                                        oldObject[@"firstName"],
                                        oldObject[@"lastName"]];
        }];
    }
};

// Tell Realm to use this new configuration object for the default Realm
[RLMRealmConfiguration setDefaultConfiguration:config];

// Now that we've told Realm how to handle the schema change, opening the realm
// will automatically perform the migration
RLMRealm *realm = [RLMRealm defaultRealm];

Swift

swift

// In version 2, the Person model has one
// combined field for the full name and age as a Int.
// A manual migration will be required to convert from
// version 1 to this version.
class Person: Object {
    @Persisted var fullName = ""
    @Persisted var age = 0
}

swift

// In application(_:didFinishLaunchingWithOptions:)
let config = Realm.Configuration(
    schemaVersion: 2, // Set the new schema version.
    migrationBlock: { migration, oldSchemaVersion in
        if oldSchemaVersion < 2 {
            // The enumerateObjects(ofType:_:) method iterates over
            // every Person object stored in the Realm file to apply the migration
            migration.enumerateObjects(ofType: Person.className()) { oldObject, newObject in
                // combine name fields into a single field
                let firstName = oldObject!["firstName"] as? String
                let lastName = oldObject!["lastName"] as? String
                newObject!["fullName"] = "\(firstName!) \(lastName!)"
            }
        }
    }
)

// Tell Realm to use this new configuration object for the default Realm
Realm.Configuration.defaultConfiguration = config

// Now that we've told Realm how to handle the schema change, opening the file
// will automatically perform the migration
let realm = try! Realm()

Later, the developer decides that the age field should be of type String rather than Int and updates the schema.

To migrate the realm to conform to the updated Person schema, the developer sets the realm's schema version to 3 and adds a conditional to the migration function so that the function defines how to migrate from any previous version to the new one.

Objective-C

objectivec

// In version 3, the Person model has one
// combined field for the full name and age as a String.
// A manual migration will be required to convert from
// version 2 to this version.
@interface Person : RLMObject
@property NSString *fullName;
@property NSString *age;
@end

@implementation Person
+ (NSArray<NSString *> *)requiredProperties {
    return @[@"fullName", @"age"];
}
@end

objectivec

RLMRealmConfiguration *config = [[RLMRealmConfiguration alloc] init];
// Set the new schema version
config.schemaVersion = 3;
config.migrationBlock = ^(RLMMigration * _Nonnull migration, uint64_t oldSchemaVersion) {
    if (oldSchemaVersion < 2) {
        // Previous Migration.
        [migration enumerateObjects:[Person className]
                            block:^(RLMObject * _Nullable oldObject, RLMObject * _Nullable newObject) {
            newObject[@"fullName"] = [NSString stringWithFormat:@"%@ %@",
                                        oldObject[@"firstName"],
                                        oldObject[@"lastName"]];
        }];
    }
    if (oldSchemaVersion < 3) {
        // New Migration
        [migration enumerateObjects:[Person className]
                            block:^(RLMObject * _Nullable oldObject, RLMObject * _Nullable newObject) {
            // Make age a String instead of an Int
            newObject[@"age"] = [oldObject[@"age"] stringValue];
        }];
    }
};

// Tell Realm to use this new configuration object for the default Realm
[RLMRealmConfiguration setDefaultConfiguration:config];

// Now that we've told Realm how to handle the schema change, opening the realm
// will automatically perform the migration
RLMRealm *realm = [RLMRealm defaultRealm];

Swift

swift

// In version 3, the Person model has one
// combined field for the full name and age as a String.
// A manual migration will be required to convert from
// version 2 to this version.
 class Person: Object {
    @Persisted var fullName = ""
    @Persisted var age = "0"
 }

swift

// In application(_:didFinishLaunchingWithOptions:)
let config = Realm.Configuration(
    schemaVersion: 3, // Set the new schema version.
    migrationBlock: { migration, oldSchemaVersion in
        if oldSchemaVersion < 2 {
            // Previous Migration.
            migration.enumerateObjects(ofType: Person.className()) { oldObject, newObject in
                let firstName = oldObject!["firstName"] as? String
                let lastName = oldObject!["lastName"] as? String
                newObject!["fullName"] = "\(firstName!) \(lastName!)"
            }
        }
        if oldSchemaVersion < 3 {
            // New Migration.
            migration.enumerateObjects(ofType: Person.className()) { oldObject, newObject in
                // Make age a String instead of an Int
                newObject!["age"] = "\(oldObject!["age"] ?? 0)"
            }
        }
    }
)

// Tell Realm to use this new configuration object for the default Realm
Realm.Configuration.defaultConfiguration = config

// Now that we've told Realm how to handle the schema change, opening the file
// will automatically perform the migration
let realm = try! Realm()

Tip: Avoid nesting or otherwise skipping if (oldSchemaVersion < X) statements in migration blocks. This ensures that all updates can be applied in the correct order, no matter which schema version a client starts from. The goal is to define migration logic which can transform data from any outdated schema version to match the current schema.

Convert from Object to EmbeddedObject

Embedded objects cannot exist independently of a parent object. When changing an Object to an EmbeddedObject, the migration block must ensure that every embedded object has exactly one backlink to a parent object. Having no backlinks or multiple backlinks raises the following exceptions:

At least one object does not have a backlink (data would get lost).

At least one object does have multiple backlinks.

Seealso: Define an Embedded Object Property

Additional Migration Examples

Please check out the additional migration examples on the realm-swift repo.