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我有这个查询应该得到一个消息列表及其发件人信息及其收件人列表（考虑groupID和senderID函数参数）：

(for {
    groupUsers <- Users.users if targets.groupID === groupID
    mRecipients <- recipients if mRecipients.targetID === groupUsers.id
    message <- messages if message.senderID === groupUsers.id || message.id === mRecipients.messageID
    sender <- Users.users if sender.id === m.senderID
} yield (message, sender ,mRecipients)).list.map{ result => new FullMessage(result ._1, result ._2, result ._3) }

但是结果，当我想要一个(Message, User, List[Recipient])的元组时，我得到了一个(Message, User, Recipient)的元组。我如何设法得到这样的结果？

谢谢你们。

4

1 回答 1

1

好吧...代码将根据您编写的内容而不是您想要的内容生成输出。专注于这部分...

yield (message, sender ,mRecipients)

你正在产生一个元组(message, sender, mRecipents)

你在 for 理解中做了这个

mRecipients <- recipients if mRecipients.targetID === targetID
message <- messages if message.senderID === senderID || message.id === mRecipients.messageID
sender <- Users.users if sender.id === m.senderID

在这里，每个表达式 like 都recipients if mRecipients.targetID === targetID计算为一个Querymonad。

要了解这有什么问题，它与以下非常相似，让我们想想当我们这样做时会发生什么，

scala>:pa
// Entering paste mode (ctrl-D to finish

for{
  i <- List( 1, 2, 3 )
  j <- List( 1, 2, 3 )
  k <- List( 1, 2, 3 )
} yield( i, j, k )

// Exiting paste mode, now interpreting.

res1: List[(Int, Int, Int)] = List((1,1,1), (1,1,2), (1,1,3), (1,2,1),
(1,2,2), (1,2,3), (1,3,1), (1,3,2), (1,3,3), (2,1,1), (2,1,2), (2,1,3),
(2,2,1), (2,2,2), (2,2,3), (2,3,1), (2,3,2), (2,3,3), (3,1,1), (3,1,2),
(3,1,3), (3,2,1), (3,2,2), (3,2,3), (3,3,1), (3,3,2), (3,3,3))

简单来说，for comprehensions做一些类似于打开monad并获取其中值的迭代器的事情......然后它将产生的值包装到monad再次中。

所以，如果你做类似的事情，

scala>:pa
// Entering paste mode (ctrl-D to finish

for{
  l <- Some( List( 1, 2, 3 ) )
  i <- Some( 1 )
} yield ( l, i ) 

// Exiting paste mode, now interpreting.
res1: Option[(List[Int], Int)] = Some((List(1, 2, 3),1))

另外...您的数据模型感觉有点不对。

什么是recipients？？告诉我，也许我将能够以更好、更有效的方式解决您的问题。

但是考虑到我对您的数据模型的猜测，现在如果您希望以正确的方式列出收件人列表，您将不得不执行以下操作，

// List[ ( Message, List[ Recipients] ) ]
val messageAndRecipentsList = ( for {

  // First get all possible recipents
  possibleRecipient <- recipients if possibleRecipient.targetID === targetID

  // Now get all relevant messages for sender and possibleRecipeint
  message <- messages if message.senderID === senderID || message.id === possibleRecipient.messageID

  // Now get list of recipents for each of these messages
  messageRecipients <- for {
    messageRecipient <- recipients if messageRecipient.messageID === message.id
  } yield ( messageRecipient ).list

} yield ( message, messageRecipients ) ).list

// Now lets add sender for each of these messages, I am guessing sender
// should be exactly one for each sendorId. So... instead of querying for
// same user for every message lets just query it once and add to every
// tuple of the above list,

val sender = ( for {
  sender <- Users.users if sender.id === senderID
} yield sender ).list.head


val finalList = messageAndRecipentsList.map ( {
  case ( message, msaageRecipents ) => new FullMessage( message, sender, messageRecipients )
} )
于 2015-03-12T09:16:04.677 回答