Some extension functions with room: Requires Export schema Read this section https://medium.com/google-developers/testing-room-migrations-be93cdb0d975#6872

DbSchema.kt

import com.squareup.moshi.Json


data class DbSchema(@field:Json(name = "formatVersion") val formatVersion: Int? = 0, @field:Json(
		name = "database") val database: Database? = Database())

data class Database(@field:Json(name = "version") val version: Int = 0, @field:Json(name = "identityHash") val identityHash: String? = "", @field:Json(
		name = "entities") val entities: List<Entity?>? = listOf(), @field:Json(name = "setupQueries") val setupQueries: List<String?>? = listOf())

data class Entity(@field:Json(name = "tableName") val tableName: String? = "", @field:Json(name = "createSql") val createSql: String? = "", @field:Json(
		name = "fields") val fields: List<Field?>? = listOf(), @field:Json(name = "primaryKey") val primaryKey: PrimaryKey? = PrimaryKey(), @field:Json(
		name = "indices") val indices: List<Indice?>? = listOf(), @field:Json(name = "foreignKeys") val foreignKeys: List<ForeignKey?>? = listOf())

data class Indice(@field:Json(name = "name") val name: String? = "", @field:Json(name = "unique") val unique: Boolean? = false, @field:Json(
		name = "columnNames") val columnNames: List<String?>? = listOf(), @field:Json(name = "createSql") val createSql: String? = "")

data class PrimaryKey(@field:Json(name = "columnNames") val columnNames: List<String?>? = listOf(), @field:Json(
		name = "autoGenerate") val autoGenerate: Boolean? = false)

data class Field(@field:Json(name = "fieldPath") val fieldPath: String? = "", @field:Json(name = "columnName") val columnName: String? = "", @field:Json(
		name = "affinity") val affinity: String? = "", @field:Json(name = "notNull") val notNull: Boolean? = false)

data class ForeignKey(@Json(name = "table") val table: String? = "", @Json(name = "onDelete") val onDelete: String? = "", @Json(
		name = "onUpdate") val onUpdate: String? = "", @Json(name = "columns") val columns: List<String?>? = listOf(), @Json(
		name = "referencedColumns") val referencedColumns: List<String?>? = listOf())

Examples.kt

//Callback DSLs
Room.databaseBuilder(application,
            AppDatabase::class.java,
            "Demo.db").onCreate { db: SupportSQLiteDatabase ->

    }.onOpen { db: SupportSQLiteDatabase -> }.addMigration(oldVersion = 1, newVersion = 2) {db:SupportSQLiteDatabase->
				val migrationUtils = MigrationUtils(db,
						context = application.applicationContext,
						moshi = moshi)

				val migrationStatements = migrationUtils.getMigrationStatements("com.zoomi.globalretailer.db.AppDatabase/2.json")

				migrationStatements.forEach {
					db.execSQL(it)
				}
			}.build()

//Dagger example from Production App
	@Provides
	@AppScope
	fun providesAppDB(application: GlobalRetailerApp, moshi: Moshi) = Room.databaseBuilder(
			application,
			AppDatabase::class.java,
			"Demo.db").addMigrationList(MigrationUtils(application.applicationContext,
			moshi).getMigrations()).build()

MigrationUtils.kt

//To make this class work, schema must be exported.
// To export the schema, follow Prerequisites section from below link
//https://medium.com/google-developers/testing-room-migrations-be93cdb0d975#6872

class MigrationUtils(val context: Context, val moshi: Moshi) {


	private fun getSchemas(): MutableList<DbSchema> {

		Timber.tag("Schemas")
		val name = AppDatabase::class.java.name
		Timber.d(name)
		val path = [email protected](name)

		Timber.tag("Schemas")
		Timber.d(path.joinToString())
		val tableInfoArray = mutableListOf<DbSchema>()
		path.map { getTableInfoFromJson("$name/$it") }.forEach {
			it?.let {
				tableInfoArray += it
			}
		}
		return tableInfoArray

	}

	private fun getTableJson(fileName: String): String = [email protected](
			fileName = fileName)

	private fun getTableInfoFromJson(fileName: String): DbSchema? {
		val jsonString = getTableJson(fileName)
		return jsonString.fromJson<DbSchema>(moshi)
	}


	fun getMigrations(): List<Migration> {

		val migrations = mutableListOf<Migration>()
		val dbSchemas = getSchemas()
		dbSchemas.reduce { acc, dbSchema ->
			if (acc.database != null && dbSchema.database != null) {
				migrations += getMigrationStatement(acc.database, dbSchema.database)
			}
			dbSchema
		}

		return migrations


	}

	private fun getMigrationStatement(first: Database, second: Database): Migration {

		val statementsToExecute = mutableListOf<String>()
		val firstEntities = first.entities ?: listOf()
		val secondEntities = second.entities ?: listOf()

		for (entity in secondEntities) {

			entity?.let { (tableName, _, fields, _, indices) ->

				val firstEntity = firstEntities.first { tableName == it?.tableName }


				//Compare Columns
				val firstColumns = firstEntity?.fields ?: listOf()
				val secondColumns = fields ?: listOf()

				val newColumns = secondColumns.filterNot { firstColumns.contains(it) }

				//Add new Columns
				for (column in newColumns) {
					column?.let {
						val constraint = if (it.notNull == true) "NOT NULL DEFAULT ${getDefaultValue(
								it.affinity)}" else ""
						val migrationStatement = "Alter table '$tableName' Add column '${it.columnName}' ${it.affinity} $constraint"
						Timber.d(migrationStatement)
						statementsToExecute += migrationStatement
					}
				}

				//Indices
				val firstIndices = firstEntity?.indices ?: listOf()
				val secondIndices = indices ?: listOf()

				val newIndices = secondIndices.filterNot { firstIndices.contains(it) }

				for (newIndex in newIndices) {
					newIndex?.let {
						val migrationStatementDrop = "Drop Index ${it.name}"
						val migrationStatementReCreate = it.createSql ?: ""

						Timber.d(migrationStatementDrop)
						Timber.d(migrationStatementReCreate)

						statementsToExecute += migrationStatementDrop
						statementsToExecute += migrationStatementReCreate
					}
				}
			}
		}

		return object : Migration(first.version, second.version) {
			override fun migrate(database: SupportSQLiteDatabase) {
				try {
					for (statement in statementsToExecute) {
						Timber.tag("Statement")
						Timber.w(statement)
						database.execSQL(statement)
					}
				} catch (e: Exception) {
					e.printStackTrace()
				}
			}

		}

	}

	private fun getDefaultValue(affinity: String?): String = when (affinity ?: "") {
		"NUMBER" -> "0"
		"TEXT" -> "''"
		else -> "''"
	}


}

RoomDbExtensions.kt

inline fun <T : RoomDatabase?> RoomDatabase.Builder<T>.callback(crossinline onCreateFun: (SupportSQLiteDatabase) -> Unit = {},
                                                                crossinline onOpenFun: (SupportSQLiteDatabase) -> Unit = {}): RoomDatabase.Builder<T> {

	addCallback(object : RoomDatabase.Callback() {
		override fun onCreate(db: SupportSQLiteDatabase) {
			onCreateFun(db)
			super.onCreate(db)
		}

		override fun onOpen(db: SupportSQLiteDatabase) {
			onOpenFun(db)
			super.onOpen(db)
		}
	})
	return this
}

inline fun <T : RoomDatabase?> RoomDatabase.Builder<T>.onCreate(crossinline onCreateFun: (SupportSQLiteDatabase) -> Unit = {}): RoomDatabase.Builder<T> = callback(
		onCreateFun = onCreateFun)

inline fun <T : RoomDatabase?> RoomDatabase.Builder<T>.onOpen(crossinline onOpenFun: (SupportSQLiteDatabase) -> Unit = {}): RoomDatabase.Builder<T> = callback(
		onOpenFun = onOpenFun)



inline fun <T : RoomDatabase> RoomDatabase.Builder<T>.addMigration(oldVersion: Int,
                                                                   newVersion: Int,
                                                                   crossinline migrateFun: (SupportSQLiteDatabase) -> Unit = {}): RoomDatabase.Builder<T> {

	return addMigrations(object : Migration(oldVersion, newVersion) {
		override fun migrate(database: SupportSQLiteDatabase) {
			migrateFun(database)
		}
	})
}

 fun <T : RoomDatabase> RoomDatabase.Builder<T>.addMigrationList(migrations: List<Migration>): RoomDatabase.Builder<T> = addMigrations(*migrations.toTypedArray())