Migrating Realm SDK database - Local Database

This is a follow-up article in the Getting Started Series.
In this article, we learn how to modify/migrate Realm local database schema.

Introduction

As you add and change application features, you need to modify database schema, and the need for
migrations arises, which is very important for a seamless user experience.

By the end of this article, you will learn:

1. How to update database schema post-production release on play store.
2. How to migrate user data from one schema to another.

Before we get down to business, let's quickly recap how we set Realm in our application.

const val REALM_SCHEMA_VERSION: Long = 1
const val REALM_DB_NAME = "rMigrationSample.db"

fun setupRealm(context: Context) {
    Realm.init(context)

    val config = RealmConfiguration.Builder()
        .name(REALM_DB_NAME)
        .schemaVersion(REALM_SCHEMA_VERSION)
        .build()

    Realm.setDefaultConfiguration(config)
}

Doing migration in Realm is very straightforward and simple. The high-level steps for the successful
migration of any database are:

1. Update the database version.
2. Make changes to the database schema.
3. Migrate user data from old schema to new.

Update the Database Version

This is the simplest step, which can be done by incrementing the version of
REALM_SCHEMA_VERSION, which notifies Relam about database changes. This, in turn, runs triggers
migration, if provided.

To add migration, we use the migration function available in RealmConfiguration.Builder, which
takes an argument of RealmMigration, which we will review in the next step.

val config = RealmConfiguration.Builder()
    .name(REALM_DB_NAME)
    .schemaVersion(REALM_SCHEMA_VERSION)
    .migration(DBMigrationHelper())
    .build()

Make Changes to the Database Schema

In Realm, all the migration-related operation has to be performed within the scope
of RealmMigration.

class DBMigrationHelper : RealmMigration {

    override fun migrate(realm: DynamicRealm, oldVersion: Long, newVersion: Long) {
        migration1to2(realm.schema)
        migration2to3(realm.schema)
        migration3to4(realm.schema)
    }

    private fun migration3to4(schema: RealmSchema?) {
        TODO("Not yet implemented")
    }

    private fun migration2to3(schema: RealmSchema?) {
        TODO("Not yet implemented")
    }

    private fun migration1to2(schema: RealmSchema) {
        TODO("Not yet implemented")
    }
}

To add/update/rename any field:

private fun migration1to2(schema: RealmSchema) {
    val userSchema = schema.get(UserInfo::class.java.simpleName)
    userSchema?.run {
        addField("phoneNumber", String::class.java, FieldAttribute.REQUIRED)
        renameField("phoneNumber", "phoneNo")
        removeField("phoneNo")
    }
}

Migrate User Data from Old Schema to New

All the data transformation during migration can be done with transform function with the help
of set and get methods.

private fun migration2to3(schema: RealmSchema) {
    val userSchema = schema.get(UserInfo::class.java.simpleName)
    userSchema?.run {
        addField("fullName", String::class.java, FieldAttribute.REQUIRED)
        transform {
            it.set("fullName", it.get<String>("firstName") + it.get<String>("lastName"))
        }
    }
}

In the above snippet, we are setting the default value of fullName by extracting the value from
old data, like firstName and lastName.

We can also use transform to update the data type.

val personSchema = schema!!["Person"]

// Change type from String to int
schema["Pet"]?.run {
    addField("type_tmp", Int::class.java)
    transform { obj ->
        val oldType = obj.getString("type")
        if (oldType == "dog") {
            obj.setLong("type_tmp", 1)
        } else if (oldType == "cat") {
            obj.setInt("type_tmp", 2)
        } else if (oldType == "hamster") {
            obj.setInt("type_tmp", 3)
        }
    }
    removeField("type")
    renameField("type_tmp", "type")
}

In case you want to delete the complete Realm, you can use deleteRealmIfMigrationNeeded with
RealmConfiguration.Builder, but that should be considered as the last resort.

Thank you for reading! If you have any queries or comments, you can share them on the MongoDB Realm forum or tweet
me @codeWithMohit.

In the next article, we will discuss how to migrate the Realm database with sync.

If you have an iOS app, do check out the iOS tutorial
on Realm iOS Migration.