title: SubtractByKey and keep rejected values
body: <p>I was playing around with spark and I am getting stuck with something that seems foolish.</p>
  
  <p>Let's say we have two RDD:</p>
  
  <p><code>rdd1 = {(1, 2), (3, 4), (3, 6)}</code> </p>
  
  <p><code>rdd2 = {(3, 9)}</code> </p>
  
  <p>if I am doing <code>rdd1.substrackByKey(rdd2)</code> , I will get <code>{(1, 2)}</code> wich is perfectly fine. But I also want to save the rejected values <code>{(3,4),(3,6)}</code> to another RDD, is there a prebuilt function in spark or an elegant way to do this?</p>
  
  <p>Please keep in mind that I am new with Spark, any help will be appreciated, thanks. </p>