implement 2-stage mapReduce paginator

modules/db/src/main/PaginatorAdapter.scala

@@ -33,26 +33,40 @@ final class Adapter[A: BSONDocumentReader](
       .list[A](length, readPreference)
 }
 
+/*
+ * because mongodb mapReduce doesn't support `skip`, slice requires to queries.
+ * The first one gets the IDs with `skip`.
+ * The second one runs the mapReduce on these IDs.
+ * This avoid running mapReduce on many unnecessary docs.
+ * NOTE: Requires string ID.
+ */
 final class MapReduceAdapter[A: BSONDocumentReader](
     collection: Coll,
     selector: Bdoc,
+    sort: Bdoc,
     runCommand: RunCommand,
-    command: MapReduceAdapter.Command,
+    command: Bdoc,
     readPreference: ReadPreference = ReadPreference.primary
 ) extends AdapterLike[A] {
 
   def nbResults: Fu[Int] = collection.countSel(selector, readPreference)
 
   def slice(offset: Int, length: Int): Fu[List[A]] =
-    runCommand(
-      command(offset) ++ $doc("limit" -> (offset + length)),
-      readPreference
-    ) map { res =>
-        res.getAs[List[Bdoc]]("results").??(_ map implicitly[BSONDocumentReader[A]].read)
+    collection.find(selector, $id(true))
+      .sort(sort)
+      .skip(offset)
+      .list[Bdoc](length, readPreference)
+      .dmap { _ flatMap { _.getAs[BSONString]("_id") } }
+      .flatMap { ids =>
+        runCommand(
+          $doc(
+            "mapreduce" -> collection.name,
+            "query" -> $inIds(ids),
+            "out" -> $doc("inline" -> true)
+          ) ++ command,
+          readPreference
+        ) map { res =>
+            res.getAs[List[Bdoc]]("results").??(_ map implicitly[BSONDocumentReader[A]].read)
+          }
       }
 }
-
-object MapReduceAdapter {
-  // offset -> doc
-  type Command = Int => Bdoc
-}

modules/study/src/main/StudyMultiBoard.scala

@@ -36,27 +36,23 @@ final class StudyMultiBoard(
      * return the last mainline node.
      * Else, return the root node without its children.
      */
-    val command: MapReduceAdapter.Command = offset => $doc(
-      "mapreduce" -> chapterColl.name,
-      "map" -> """var node = this.root, child, result = {name:this.name,orientation:this.setup.orientation,tags:this.tags};
+    Paginator(
+      adapter = new MapReduceAdapter[ChapterPreview](
+        collection = chapterColl,
+        selector = selector,
+        sort = $sort asc "order",
+        runCommand = runCommand,
+        command = $doc(
+          "map" -> """var node = this.root, child, result = {name:this.name,orientation:this.setup.orientation,tags:this.tags};
 if (this.tags.filter(t => t.indexOf('White:') === 0 || t.indexOf('Black:') === 0).length === 2) {
   while(child = node.n[0]) { node = child };
 }
 result.fen = node.f;
 result.uci = node.u;
 emit(this._id, result)""",
-      "reduce" -> """function(key, values) { return key; }""",
-      "out" -> $doc("inline" -> true),
-      "query" -> selector,
-      "sort" -> $sort.asc("order"),
-      "jsMode" -> true
-    )
-    Paginator(
-      adapter = new MapReduceAdapter[ChapterPreview](
-        collection = chapterColl,
-        selector = selector,
-        runCommand = runCommand,
-        command = command
+          "reduce" -> """function(key, values) { return key; }""",
+          "jsMode" -> true
+        )
       ),
       currentPage = page,
       maxPerPage = maxPerPage