> For the complete MongoDB documentation index, see www.mongodb.com/docs/llms.txt # Change an Object Model - Swift SDK Atlas Device SDKs are deprecated. Refer to the deprecation page for details. **Note: Modify Schema Properties of a Synced Realm** The following page demonstrates how to modify schema properties of a local realm. Learn how to [modify schema properties of a synced realm.](https://www.mongodb.com/docs/atlas/app-services/sync/data-model/update-schema.md#std-label-synced-schema-overview) ## Overview When you update your object schema, you must increment the schema version and perform a migration. **See also:** This page provides general Swift and Objective-C migration examples. If you are using Realm with SwiftUI, see the [SwiftUI-specific migration examples.](https://www.mongodb.com/docs/atlas/device-sdks/sdk/swift/swiftui/model-data/change-an-object-model.md#std-label-swiftui-realm-migrations) 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: Bypass Migration During Development** When developing or debugging your application, you may prefer to delete the realm instead of migrating it. Use the [deleteRealmIfMigrationNeeded](https://www.mongodb.com/docs/realm-sdks/swift/latest/Structs/Realm/Configuration.html#/s:10RealmSwift0A0V13ConfigurationV06deleteA17IfMigrationNeededSbvp) 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](https://www.mongodb.com/docs/atlas/device-sdks/sdk/swift/model-data.md#std-label-ios-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: Increment Versions Monotonically** 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.](https://www.mongodb.com/docs/atlas/device-sdks/sdk/swift/model-data.md#std-label-ios-object-models-and-schemas) ### Migrations A **local migration** is a migration for a realm that does not automatically [Sync](https://www.mongodb.com/docs/atlas/app-services/sync.md#std-label-sync) with another realm. 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](https://www.mongodb.com/docs/atlas/device-sdks/sdk/swift/model-data/change-an-object-model.md#std-label-ios-perform-a-schema-migration). 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 *)requiredProperties { return @[@"firstName", @"lastName", @"age"]; } @end ``` 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 *)requiredProperties { return @[@"firstName", @"lastName", @"email", @"age"]; } @end ``` 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]; ``` ### 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 *)requiredProperties { return @[@"firstName", @"lastName", @"age"]; } @end ``` 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 *)requiredProperties { return @[@"firstName", @"lastName"]; } @end ``` 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]; ``` **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: )` 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:)](https://www.mongodb.com/docs/realm-sdks/swift/latest/Structs/Migration.html#/s:10RealmSwift9MigrationV14renameProperty6onType4from2toySS_S2StF) method. Realm applies any new nullability or indexing settings during the rename operation. **Example:** Rename `age` to `yearsSinceBirth` within a [migrationBlock.](https://www.mongodb.com/docs/realm-sdks/swift/latest/Structs/Realm/Configuration.html#/s:10RealmSwift0A0V13ConfigurationV14migrationBlockyAA9MigrationV_s6UInt64VtcSgvp) ### 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"]; } }; ``` ### Modify Properties **Tip:** You can use the [deleteRealmIfMigrationNeeded](https://www.mongodb.com/docs/realm-sdks/swift/latest/Structs/Realm/Configuration.html#/s:10RealmSwift0A0V13ConfigurationV06deleteA17IfMigrationNeededSbvp) 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](https://www.mongodb.com/docs/realm-sdks/swift/latest/Structs/Realm/Configuration.html#/s:10RealmSwift0A0V13ConfigurationV14migrationBlockyAA9MigrationV_s6UInt64VtcSgvp) property of the [Configuration](https://www.mongodb.com/docs/realm-sdks/swift/latest/Structs/Realm/Configuration.html) when [opening a realm.](https://www.mongodb.com/docs/atlas/device-sdks/sdk/swift/realm-files/configure-and-open-a-realm.md#std-label-ios-configure-and-open-a-realm) The migration block receives a [Migration object](https://www.mongodb.com/docs/realm-sdks/swift/latest/Structs/Migration.html) that you can use to perform the migration. You can use the Migration object's [enumerateObjects(ofType:\_:)](https://www.mongodb.com/docs/realm-sdks/swift/latest/Structs/Migration.html#/s:10RealmSwift9MigrationV16enumerateObjects6ofType_ySS_yAA13DynamicObjectCSg_AHtXEtF) 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 *)requiredProperties { return @[@"firstName", @"lastName", @"age"]; } @end ``` 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 *)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]; ``` 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 *)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]; ``` **Tip: Linear Migrations** 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](https://www.mongodb.com/docs/atlas/device-sdks/sdk/swift/model-data/relationships.md#std-label-ios-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: ```text At least one object does not have a backlink (data would get lost). ``` ```text At least one object does have multiple backlinks. ``` **See also:** [Define an Embedded Object Property](https://www.mongodb.com/docs/atlas/device-sdks/sdk/swift/model-data/relationships.md#std-label-ios-embedded-objects) ## Additional Migration Examples Please check out the additional migration examples on the [realm-swift repo.](https://github.com/realm/realm-swift/tree/master/examples/ios/swift/Migration)