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Update a Realm Object Schema - Flutter SDK

On this page

  • Schema Version
  • Automatically Update Schema
  • Add a Property
  • Delete a Property
  • Manually Migrate Schema
  • Delete an Object Type
  • Rename a Property
  • Other Migration Tasks
  • Updating the Schema of a Synced Realm

You can change the schema of a Realm object after you first create it. Depending on the type of changes you make to the schema, the changes can be either automatically applied or require a manual update to the new schema. Manual schema updates are called migrations in Realm.

You can automatically update a Realm object schema when you add or delete a property from a Realm object model. For more information, refer to the Automatically Update Schema section.

You must perform a manual migration for all other schema changes. For more information, refer to the Manually Migrate Schema section.

Tip

Bypass Migration During Development

When developing or debugging your application, you may prefer to delete the realm instead of migrating it. Use the LocalConfiguration.shouldDeleteIfMigrationNeeded property to delete the database automatically when a schema mismatch requires a migration.

Important

Modify Schema Properties of a Synced Realm

The contents on this page only apply to local realms. Schema migration functions differently for Realm using Atlas Device Sync to synchronize data with MongoDB Atlas. Refer to the Updating the Schema of a Synced Realm section.

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 ascending integers that you can optionally 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.

Manual migrations must update a realm to a higher schema version. Realm throws 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.

Automatically Update Schema

Realm can automatically migrate added and deleted properties. You must update the schema version when you make these changes.

Add a Property

To add a property to a schema:

  1. Add the new property to the object's RealmModel class.

  2. Regenerate the RealmObject.

  3. Set a schema version to the realm's Configuration object.

Example

A realm using schema version 1 has a Person object type with a firstName, and lastName property. The developer decides to add an age property to the _Person RealmModel class.

To change the realm to conform to the updated Person schema, the developer sets the realm's schema version to 2.

@RealmModel()
class _Person {
  late String firstName;
  late String lastName;
  late int age;
}
final config = Configuration.local([Person.schema], schemaVersion: 2);
final realm = Realm(config);

Delete a Property

To delete a property from a schema:

  1. Remove the property from the object's RealmModel class.

  2. Regenerate the RealmObject.

  3. In the realm's configuration, include the regenerated RealmObject.schema and increment the schemaVersion.

Deleting a property does not impact existing objects.

Example

A realm using schema version 1 has a Person object type with a weight property. The developer decides to remove the property from the schema.

To migrate the realm to conform to the updated Person schema, the developer sets the realm's schema version to 2.

final config = Configuration.local([Person.schema], schemaVersion: 2);
final realm = Realm(config);

Manually Migrate Schema

For more complex schema updates, Realm requires you to manually migrate old instances of a given object to the new schema.

When you open the realm with the updated schema, you must do the following in the realm's Configuration:

  • Increment the schemaVersion property.

  • Define the migration logic in a migrationCallback property of a realm's Configuration. The migration callback has the following parameters:

    • migration: A Migration instance with access to the current realm, the realm that you're migrating to, and methods to help with the migration operation.

    • oldSchemaVersion: The number of the previous schema version of the realm on the device.

The following sections explain how to perform various migration operations.

Delete an Object Type

To delete all objects of a type from your realm, pass a string representation of the object schema's name to Migration.deleteType().

This is useful if the previous version of a schema has a Realm object type, but the new version of the schema does not.

final configWithoutPerson = Configuration.local([Car.schema],
    schemaVersion: 2,
    migrationCallback: ((migration, oldSchemaVersion) {
  // Between v1 and v2 we removed the Person type
  migration.deleteType('Person');
}));
final realmWithoutPerson = Realm(configWithoutPerson);

Rename a Property

Rename a schema property with Migration.renameProperty().

final configWithRenamedAge =
    Configuration.local([Person.schema, Car.schema],
        schemaVersion: 2,
        migrationCallback: ((migration, oldSchemaVersion) {
  // Between v1 and v2 we renamed the Person 'age' property to 'yearsSinceBirth'
  migration.renameProperty('Person', 'age', 'yearsSinceBirth');
}));
final realmWithRenamedAge = Realm(configWithRenamedAge);

Other Migration Tasks

To perform other realm schema migrations, use the following properties of the Migration object in your migration callback function:

  • Migration.oldRealm: The realm as it existed just before the migration with the previous schema version. You must use the oldRealm's dynamic API to access its objects because you cannot use standard type-based queries. The dynamic API lets you find Realm objects by a string representation of their class name.

  • Migration.newRealm: The realm as it exists after the migration. By the end of the migration callback, you must migrate all data affected by the schema update from oldRealm into newRealm. Any data affected by the schema update that is not migrated will be lost.

To find instances of an object in an old realm in the new realm, use Migration.findInNewRealm(). To access the properties of objects from the old schema, use the RealmObjectBase.dynamic API.

final configWithChanges = Configuration.local([Person.schema, Car.schema],
    schemaVersion: 2,
    migrationCallback: ((migration, oldSchemaVersion) {
  // Dynamic query for all Persons in previous schema
  final oldPeople = migration.oldRealm.all('Person');
  for (final oldPerson in oldPeople) {
    // Find Person instance in the updated realm
    final newPerson = migration.findInNewRealm<Person>(oldPerson);
    if (newPerson == null) {
      // That person must have been deleted, so nothing to do.
      continue;
    }
    // Use dynamic API to get properties from old schema and use in the
    // new schema
    newPerson.fullName = oldPerson.dynamic.get<String>("firstName") +
        " " +
        oldPerson.dynamic.get<String>("lastName");
    // convert `id` from ObjectId to String
    final oldId = oldPerson.dynamic.get<ObjectId>("id");
    newPerson.id = oldId.toString();
  }
}));
final realmWithChanges = Realm(configWithChanges);

Updating the Schema of a Synced Realm

Updating the schema of a synced realm is a separate process from updating the schema of a local-only realm.

Synced realms do not have schema versions and automatically migrate objects to the latest schema. Synced realms only support non-breaking schema changes.

Learn how to modify schema properties of a synced realm.

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