Android Room Persistence library and Kotlin

Usually in project build.gradle I define the dependencies versions:

ext {    buildToolsVersion = '25.0.2'    supportLibVersion = '25.3.1'    espressoVersion = '2.2.2'    archRoomVersion = '1.0.0-alpha1'}

so in app build.gradle the dependencies look like:

dependencies {    compile fileTree(dir: 'libs', include: ['*.jar'])    compile "org.jetbrains.kotlin:kotlin-stdlib-jre7:$kotlin_version"    compile "com.android.support:appcompat-v7:${rootProject.supportLibVersion}"    compile "android.arch.persistence.room:runtime:${rootProject.archRoomVersion}"    annotationProcessor "android.arch.persistence.room:compiler:${rootProject.archRoomVersion}"    kapt "android.arch.persistence.room:compiler:${rootProject.archRoomVersion}"    androidTestCompile("com.android.support.test.espresso:espresso-core:${rootProject.espressoVersion}", {        exclude group: 'com.android.support', module: 'support-annotations'    })    testCompile 'junit:junit:4.12'}

Now you can define Entities Daos and Database in Kotlin.

Database:

@Database(entities = arrayOf(User::class), version = 1)abstract class Database : RoomDatabase() {    abstract fun userDao(): UserDao}

Entity:

@Entity(tableName = "user")class User {    @PrimaryKey(autoGenerate = true)    var id: Int = 0    var name: String = ""}

Dao:

@Daointerface UserDao {    @Query("SELECT * FROM user")    fun getAll(): List<User>    @Insert    fun insertAll(vararg users: User)    @Delete    fun delete(user: User)}

NB: Query with parameters.Kotlin renames params, so the SQL query to retrieve all the emails that belong at an user via the userId is:

@Query("SELECT * FROM email "            + "INNER JOIN user ON user.id = email.userId "            + "WHERE user.id = :arg0")    fun getEmailsForUser(userId: Int): List<Email>

In my case, in build.gradle, when you have "annotationProcessor" you need to duplicate with "kapt" and it works.

compile "android.arch.persistence.room:runtime:$room_version"annotationProcessor "android.arch.persistence.room:compiler:$room_version"kapt "android.arch.persistence.room:compiler:$room_version"

Try out these steps

Step 1. Set the room_version in the project.gradle file

buildscript {    ext.kotlin_version = '1.1.51'    ext.room_version = '1.0.0-alpha9-1'...

Step 2. Apply the kotlin-kapt plugin in the app.gradle file, and this solved my issue.

apply plugin: 'com.android.application'apply plugin: 'kotlin-android'apply plugin: 'kotlin-android-extensions'apply plugin: 'kotlin-kapt'android {    compileSdkVersion 26    buildToolsVersion "26.0.1"...

Step 3. Add the kapt dependency in the app.gradle file

dependencies {    implementation fileTree(include: ['*.jar'], dir: 'libs')    implementation "android.arch.persistence.room:runtime:$room_version"    annotationProcessor "android.arch.persistence.room:compiler:$room_version"    kapt "android.arch.persistence.room:compiler:$room_version"...}