Batch updates on reactive mongo sometimes fails.

gistfile1.scala

val batchSize = 100 // for some reason we need to throttle down the import, putting everything in sometimes causes a future already completed
	
Async {
	fetch("/instrument").map { instruments =>
		Logger.debug(s"importing ${instruments.size} instruments")

		instruments.grouped(batchSize).foreach { batch =>
			res.batchInsert(Enumerator.enumerate(batch))
		}
		
		Accepted
	}
}

// above seems to be working, but if batchSize is big like more than 500 some batches failes to insert more often
// even wait for future by doing a foldLeft and map like below.  if batchSize is about 100 it seems to be ok

val batchSize = 100 // for some reason we need to throttle down the import, putting everything in sometimes causes a future already completed
	
Async {
	fetch("/instrument").flatMap { instruments =>
		Logger.debug(s"importing ${instruments.size} instruments")

		instruments.grouped(batchSize).foldLeft(Future{Accepted}) { (acc, batch) =>
			res.batchInsert(Enumerator.enumerate(batch)).map{ _ =>
				Accepted
			}
		}
		
	}
}


// The straight forward code fails, unless the instruments are less than 500.

Async {
	fetch("/instrument").flatMap { instruments =>
		Logger.debug(s"importing ${instruments.size} instruments")

		res.batchInsert(Enumerator.enumerate(instruments)).map{ _ =>
			Accepted
		}
		
	}
}

// stack trace below
java.lang.IllegalStateException: Promise already completed.
  at scala.concurrent.Promise$class.complete(Promise.scala:55)
	at scala.concurrent.impl.Promise$DefaultPromise.complete(Promise.scala:58)
	at scala.concurrent.Promise$class.failure(Promise.scala:107)
	at scala.concurrent.impl.Promise$DefaultPromise.failure(Promise.scala:58)
	at scala.concurrent.Future$$anonfun$flatMap$1.liftedTree3$1(Future.scala:283)
	at scala.concurrent.Future$$anonfun$flatMap$1.apply(Future.scala:277)
	at scala.concurrent.Future$$anonfun$flatMap$1.apply(Future.scala:274)
	at scala.concurrent.impl.CallbackRunnable.run(Promise.scala:29)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
	at java.lang.Thread.run(Thread.java:724)