don't load forum participants' usernames into the light user cache

app/controllers/ForumTopic.scala

@@ -77,10 +77,10 @@ object ForumTopic extends LilaController with ForumController {
   /**
    * Returns a list of the usernames of people participating in a forum topic conversation
    */
-  def participants(topicId: String) = Auth { implicit ctx => me =>
-    postApi.userIds(topicId) map { ids =>
-      val usernames = Env.user.lightUserApi.getList(ids.sorted).map(_.name)
-      Ok(Json.toJson(usernames))
-    }
+  def participants(topicId: String) = Auth { implicit ctx => _ =>
+    for {
+      userIds <- postApi userIds topicId
+      usernames <- lila.user.UserRepo usernamesByIds userIds
+    } yield Ok(Json.toJson(usernames.sorted))
   }
 }

modules/user/src/main/LightUserApi.scala

@@ -21,10 +21,6 @@ final class LightUserApi(coll: Coll)(implicit system: akka.actor.ActorSystem) {
   def preloadMany = cache preloadMany _
   def preloadUser(user: User) = cache.setOneIfAbsent(user.id, user.light.some)
 
-  def getList(ids: List[String]): List[LightUser] = ids flatMap sync
-
-  def usernameList(ids: List[String]): List[String] = getList(ids).map(_.name)
-
   private val cache = new Syncache[String, Option[LightUser]](
     name = "user.light",
     compute = id => coll.find($id(id), projection).uno[LightUser],

modules/user/src/main/UserRepo.scala

@@ -85,7 +85,7 @@ object UserRepo {
     coll.primitiveOne[User.ID]($id(id), F.username)
 
   def usernamesByIds(ids: List[ID]) =
-    coll.distinct[User.ID, List](F.username, $inIds(ids).some)
+    coll.distinct[String, List](F.username, $inIds(ids).some)
 
   def orderByGameCount(u1: User.ID, u2: User.ID): Fu[Option[(User.ID, User.ID)]] = {
     coll.find(