Migrating Android Room Database with Conditional Updates

Introduction

Android Room provides a powerful way to manage data storage for your app. One of the features that makes it easier to work with is database migration, which allows you to update your schema over time without affecting the existing data. However, when it comes to conditional updates, things can get a bit tricky.

In this article, we’ll explore how to perform a migration from one version of Room’s database schema to another while dealing with conditions that require updating specific rows based on certain criteria.

Understanding the Basics of Room Migration

Before we dive into the nitty-gritty of conditional updates, let’s quickly review the basics of Room migration. When you create a new migration, you’re telling Room how to update your existing database schema from one version to another.

In this example, our migrations look like this:

val MIGRATION_6_7 = object : Migration(6, 7) {
    override fun migrate(database: SupportSQLiteDatabase) {
        database.execSQL("ALTER TABLE RideEntity ADD COLUMN state INTEGER DEFAULT 0 NOT NULL")
    }
}

val MIGRATION_7_8 = object : Migration(7, 8) {
    override fun migrate(database: SupportSQLiteDatabase) {
        // The update statement we're going to discuss is in here
    }
}

The Issue with the Update Statement

The problem arises when trying to update specific rows based on certain conditions. In our example, we want to update the state column from 0 to 1 for all rows where state is currently 0.

However, when we try to execute this statement using Room’s migration mechanism, we get an error:

or OR expected, got 'TABLE'

Solving the Problem

The issue here is that we’re trying to use a raw SQL update statement, which doesn’t play well with Room’s migration system. We need to figure out how to express this update statement in a way that’s compatible with Room.

One way to do this is by using Room’s update function. This function allows us to specify the column we want to update and the new value for that column, as well as specifying any conditions that should be applied to the rows we’re updating.

Here’s an example of how you can use the update function to achieve our desired behavior:

val MIGRATION_7_8 = object : Migration(7, 8) {
    override fun migrate(database: SupportSQLiteDatabase) {
        database.update(
            "RideEntity",
            "state = CASE WHEN state = 0 THEN 1 ELSE state END",
            "state = 0"
        )
    }
}

In this statement, we’re using a CASE expression to update the state column. If the current value of state is 0, we set it to 1; otherwise, we leave the value unchanged.

Conclusion

Updating data in a Room database can be challenging, especially when you need to apply conditional updates based on certain criteria. By understanding how to use Room’s migration system and by leveraging features like the update function, you can achieve complex update scenarios without having to resort to raw SQL statements.

In this article, we’ve explored one way to perform a migration from one version of Room’s database schema to another while dealing with conditions that require updating specific rows based on certain criteria.

Last modified on 2025-05-07