title: Unable to use Completable in Room dao interface
body: <p>I'm trying to use <code>Completable</code> in Dao class on <code>@Insert</code> annotated methods but when tried to compile the project, I've got this error:</p>
  
  <pre><code>error: Methods annotated with @Insert can return either void, long, Long, long[], Long[] or List&lt;Long&gt;.
  public abstract io.reactivex.Completable insert(@org.jetbrains.annotations.NotNull()
  </code></pre>
  
  <p>Here my related codes:</p>
  
  <pre><code>@Insert
  fun insert(kanal: Kanal): Completable
  @Update
  fun update(kanal: Kanal): Completable
  @Delete
  fun delete(kanal: Kanal): Completable
  </code></pre>
  
  <p>And my dependencies:</p>
  
  <pre><code>def room_version = "1.1.1"
  implementation "android.arch.persistence.room:runtime:$room_version"
  kapt "android.arch.persistence.room:compiler:$room_version"
  implementation "android.arch.persistence.room:rxjava2:$room_version"
  implementation 'io.reactivex.rxjava2:rxkotlin:2.2.0'
  implementation 'io.reactivex.rxjava2:rxandroid:2.1.0'
  </code></pre>
  
  <p>According to this <a href="https://developer.android.com/training/data-storage/room/accessing-data#query-rxjava" rel="noreferrer">link</a> it's supported by Room.</p>