title: How return completable on room transaction Android
body: <p>I need your help please.</p>
  
  <p>I have dao interface that save some configurations:</p>
  
  <pre><code>@Dao interface ConfigDao {
      @Insert(onConflict = OnConflictStrategy.REPLACE)
      fun insert(config: Config)
  
      @Update(onConflict = OnConflictStrategy.REPLACE)
      fun update(config: Config)
  
      @Query("select * from T_CONFIG where isSelected = :isSelected")
      fun getConfig(isSelected: Boolean): Single&lt;Config&gt;
  
      @Query("select * from t_config")
      fun getConfigAll(): LiveData&lt;MutableList&lt;Config&gt;&gt;
  
      @Query("update T_CONFIG set isSelected = :isSelected where idEnvironment = :id")
      fun updateConfigById(id: String, isSelected: Boolean):Completable
  
      @Transaction
      fun updateConfigTransaction(configSelected: Config){
  
          if (configSelected.idEnvironment == Environtment.Type.PRD.toString()){
              updateConfigById(Environtment.Type.PRD.toString(), false)
              updateConfigById(Environtment.Type.DEV.toString(), true)
          }else{
              updateConfigById(Environtment.Type.PRD.toString(), true)
              updateConfigById(Environtment.Type.DEV.toString(), false)
          }
      }
  }
  </code></pre>
  
  <p>I need to know when the transaction is complete with success or error.
  I tried to implement <code>Completable</code> from <code>io.reactivex</code> but it's not possible.</p>