Handle Future[Option[T]] when traversing a List-CodePudding

  def getCommentIds(
      commentTargetId: Long,
      sortOrder: CommentOrderEnum): Future[Seq[CommentStatsBO]]

def getCommentDetail(commentId: Long): Future[Option[CommentDetailDTO]]

def getCommentListWithDetail(
      targetId: Long,
      sortOrder: CommentOrderEnum,
      page: Int): Future[Seq[CommentDetailDTO]] = {
    commentModel.getCommentIds(targetId, sortOrder).flatMap {
      commentStatsBOSeq =>
        Future.traverse(commentStatsBOSeq) { commentStatsBO =>
// commentDetail is a Future[Option[T]]
          val commentDetail = getCommentDetail(commentStatsBO.iId)
          commentDetail.map(commentOpt =>
            commentOpt
// merge the stat info into the comment detail
              .map(_.copy(replyCount = Some(commentStatsBO.replyCount)))
              .getOrElse(CommentDetailDTO))

        }

    }
  }

case class CommentDetailDTO(
    id: Long,
    author: JsObject,
    detail: CommentDetail,
replyCount: Option[Int] = None
)

Firstly, the function getCommentIds returns a sequence of CommentStatsBO, then traversing it and try to get detail for every comment. Here comes the question, getCommentDetail returns a Future which contains an option since the comment maybe not found, in this case, how to filter those ones whose option is None? I have tried getOrElse , but don't know how to define an empty object just like Json.obj() since case class doesn't support. Thanks!

CodePudding user response：

Don't try to do too many things at the same time, rather build the solution you need step by step.

If you do a simple Future.traverse using just getCommentDetail you will get a Future[Seq[Option[CommentDetailDTO]]] which then you can map and use collect with the Seq to remove the Option

def getCommentListWithDetail(
  targetId: Long,
  sortOrder: CommentOrderEnum,
  page: Int
): Future[Seq[CommentDetailDTO]] =
  commentModel.getCommentIds(targetId, sortOrder).flatMap { commentStatsBOSeq =>
    Future.traverse(commentStatsBOSeq) { commentStatsBO =>
      getCommentDetail(commentStatsBO.iId)
    } map { commentOptionalDetails =>
      commentOptionalDetails.collect {
        case Some(commentDetail) => commentDetail
      }
    }
  }

Or if you use cats, you can use traverseFilter

import cats.syntax.all._

def getCommentListWithDetail(
  targetId: Long,
  sortOrder: CommentOrderEnum,
  page: Int
): Future[Seq[CommentDetailDTO]] =
  commentModel.getCommentIds(targetId, sortOrder).flatMap { commentStatsBOSeq =>
    commentStatsBOSeq.traverseFilter(getCommentDetail)
  }