+doc #19429 initial merge of docs-dev and docs

akka-docs/rst/scala/code/docs/stream/cookbook/RecipeMultiGroupBy.scala

@@ -0,0 +1,58 @@
+package docs.stream.cookbook
+
+import akka.stream.scaladsl.{ Sink, Source }
+
+import scala.collection.immutable
+import scala.concurrent.Await
+import scala.concurrent.duration._
+
+class RecipeMultiGroupBy extends RecipeSpec {
+
+  "Recipe for multi-groupBy" must {
+
+    "work" in {
+
+      case class Topic(name: String)
+
+      val elems = Source(List("1: a", "1: b", "all: c", "all: d", "1: e"))
+      val extractTopics = { msg: Message =>
+        if (msg.startsWith("1")) List(Topic("1"))
+        else List(Topic("1"), Topic("2"))
+      }
+
+      //#multi-groupby
+      val topicMapper: (Message) => immutable.Seq[Topic] = extractTopics
+
+      val messageAndTopic: Source[(Message, Topic), Unit] = elems.mapConcat { msg: Message =>
+        val topicsForMessage = topicMapper(msg)
+        // Create a (Msg, Topic) pair for each of the topics
+        // the message belongs to
+        topicsForMessage.map(msg -> _)
+      }
+
+      val multiGroups = messageAndTopic
+        .groupBy(2, _._2).map {
+          case (msg, topic) =>
+            // do what needs to be done
+            //#multi-groupby
+            (msg, topic)
+          //#multi-groupby
+        }
+      //#multi-groupby
+
+      val result = multiGroups
+        .grouped(10)
+        .mergeSubstreams
+        .map(g => g.head._2.name + g.map(_._1).mkString("[", ", ", "]"))
+        .grouped(10)
+        .runWith(Sink.head)
+
+      Await.result(result, 3.seconds).toSet should be(Set(
+        "1[1: a, 1: b, all: c, all: d, 1: e]",
+        "2[all: c, all: d]"))
+
+    }
+
+  }
+
+}