我有一个MongoDB数据库，其中存储不同的文件，这些文件可以是png，jpg，pdf，这里有一个例子：

{
    "_id" : "id of the document",
    "metadata" : {
        "s" : "documents",
        "u" : "id of the owner",
        "d" : ""
    },
    "filename" : "El-jugador-Fedor-Dostoyevski.pdf",
    "contentType" : "application/pdf",
}

字符串
我删除了一些不相关的字段，文档的类型由contentType字段给出，我想要的是使用Scala和ReactiveMongo获得每种类型文件的计数
我已经这样做了，但是做了三次咨询，这样：

def contentTypeStats(implicit ec: ExecutionContext): Future[ContentTypesDTO] = {
    collectionFactory.collection().flatMap(collection => {

      val filterDocuments = BSONDocument("metadata.s" -> BSONDocument("$ne" -> "thumbnail"))//don't want count thumbnails, only documents
      
      val filterPNG = BSONDocument(filterDocuments, "contentType" -> "image/png")
      val filterJPG = BSONDocument(filterDocuments, "contentType" -> "image/jpeg")
      val filterPDF = BSONDocument(filterDocuments, "contentType" -> "application/pdf")

      val countPNGFuture: Future[Long] = collection.count(Some(filterPNG))
      val countJPGFuture: Future[Long] = collection.count(Some(filterJPG))
      val countPDFFuture: Future[Long] = collection.count(Some(filterPDF))

      for {
        countPNG <- countPNGFuture
        countJPG <- countJPGFuture
        countPDF <- countPDFFuture
      } yield {
        ContentTypesDTO(
          pngCount = countPNG,
          jpgCount = countJPG,
          pdfCount = countPDF
        )
      }

    })
  }

型
我只想用一个consult来做这件事，在MongoDB中，我是这样做的：

db.getCollection('myCollection').aggregate([
    {$match: {'metadata.s': {$ne: 'thumbnail'}}},
    {$group: {_id: "$contentType", count: {$sum: 1}} }
])

型
他们给了我这个：

/* 1 */
{
    "_id" : "image/png",
    "count" : 5.0
}

/* 2 */
{
    "_id" : "application/pdf",
    "count" : 9.0
}

/* 3 */
{
    "_id" : "image/jpeg",
    "count" : 8.0
}

型
我试着这样做：

def contentTypeStats(implicit ec: ExecutionContext): Future[ContentTypesDTO] = {
    collectionFactory.collection().flatMap(collection => {

      import collection.AggregationFramework._
      val result: Future[Option[BSONDocument]] = collection.aggregatorContext[BSONDocument](
        pipeline = List(
          Match(BSONDocument("metadata.s" -> BSONDocument("$ne" -> "thumbnail"))),
          Group(BSONDocument("_id" -> "$contentType"))("count" -> SumAll)
        )
      ).prepared.cursor.headOption

      result.map {
        case Some(doc) =>
          println(doc.getAsOpt[String]("_id"))//here always return None
          ContentTypesDTO(
            pngCount = doc.getAsOpt[Long]("count").getOrElse(0L),
            jpgCount = doc.getAsOpt[Long]("count").getOrElse(0L),
            pdfCount = doc.getAsOpt[Long]("count").getOrElse(0L)
          )//all have the same number
      }
    })
  }

型
该方法返回None时，要求_id和计数字段给予随机一些以前的结果（5，8，9），它应该是一种方式来访问特定计数字段的每个_id，应该是image/png或image/jpeg或application/pdf，但如果我可以得到_id