persist ratingsRDD for consistency
diff --git a/src/main/scala/DataSource.scala b/src/main/scala/DataSource.scala
index bad619c..23faa3d 100644
--- a/src/main/scala/DataSource.scala
+++ b/src/main/scala/DataSource.scala
@@ -49,7 +49,8 @@
         }
       }
       rating
-    }
+    }.cache()
+
     new TrainingData(ratingsRDD)
   }
 }