Modify an Object Schema - Android SDK¶

The following examples demonstrate how to add, delete, and modify properties in a schema. First, make the required schema change. Then, create a corresponding migration function to move data from the original schema to the updated schema.

Note

Versions Update on Realm Open

Assume that each schema change in this example occurs after an application has used each version for some amount of time. New schema version numbers only apply once you open the realm with an updated schema and explicitly specify the new version number, so in order to get to version 3, you would first need to open the app with versions 1 and 2.

A realm using schema version 0 has a Person object type:

class Person: RealmObject { // Realm schema version 0
    var firstName: String = ""
    var age: int = 0
}

Add a Property¶

The following example adds a lastName property to the original Person schema:

class Person: RealmObject { // Realm schema version 1
    var firstName: String = ""
    var lastName: String = ""
    var age: int = 0
}

Delete a Property¶

The following example uses a combined fullName property instead of the separate firstName and lastName property in the original Person schema:

class Person: RealmObject { // Realm schema version 2
    var fullName: String = ""
    var age: int = 0
}

Modify a Property Type or Rename a Property¶

The following example modifies the age property in the original Person schema by renaming it to birthday and changing the type to Date:

class Person: RealmObject { // Realm schema version 3
    var fullName: String = ""
    var birthday: Date = new Date();
}

Migration Functions¶

To migrate the realm to conform to the updated Person schema, set the realm's schema version to 3 and define a migration function to set the value of fullName based on the existing firstName and lastName properties and the value of birthday based on age:

val migration = object: RealmMigration {
    override fun migrate(realm: DynamicRealm, oldVersion: Long, newVersion: Long) {
        var version: Long = oldVersion
        // DynamicRealm exposes an editable schema
        val schema: RealmSchema = realm.schema
        // Changes from version 0 to 1: Adding lastName.
        // All properties will be initialized with the default value "".
        if (version == 0L) {
            schema.get("Person")!!
                    .addField("lastName", String::class.java, FieldAttribute.REQUIRED)
            version++
        }
        // Changes from version 1 to 2: Combining firstName/lastName into fullName
        if (version == 1L) {
            schema.get("Person")!!
                    .addField("fullName", String::class.java, FieldAttribute.REQUIRED)
                    .transform { obj: DynamicRealmObject ->
                        val name = "${obj.getString("firstName")} ${obj.getString("lastName")}"
                        obj.setString("fullName", name)
                    }
                    .removeField("firstName")
                    .removeField("lastName")
            version++
        }
        // Changes from version 2 to 3: Replace age with birthday
        if (version == 2L) {
            schema.get("Person")!!
                    .addField("birthday", Date::class.java, FieldAttribute.REQUIRED)
                    .transform { obj: DynamicRealmObject ->
                        var birthYear = Date().year - obj.getInt("age")
                        obj.setDate("birthday", Date(birthYear, 1, 1))
                    }
                    .removeField("age")
            version++
        }
    }
}
@RealmModule(classes = { Person::class.java })
class Module
val config = RealmConfiguration.Builder()
    .schemaVersion(3) // Must be bumped when the schema changes
    .migration(migration) // Migration to run instead of throwing an exception
    .build()

Tip