在流，选项和创建元组上进行模式匹配 [英] Pattern matching on Stream, Option and creating a Tuple

问题描述

我有一张桌子table1:

id: Int
externalId: Int
value: String

对于给定的externalId value可以是NULL，或者可能根本不存在.我想根据这种情况返回一个元组:

For a given externalId value can be NULL or it might not exist at all. I want to return a tuple depending on this condition:

1. 如果不存在，则返回("notExists", Nil)
2. 如果存在但NULL，则返回("existsButNull", Nil)
3. 如果存在但不存在NULL，则返回("existsWithValue", ???)

1. If it doesn't exist then return ("notExists", Nil)
2. If it exists but NULL then return ("existsButNull", Nil)
3. If it exists and not NULL then return ("existsWithValue", ???)

其中，???应该是具有给定externalId的所有记录的value列表.我尝试这样做:

where ??? should be a list of value for all records with the given externalId. I tried to do this:

DB.withConnection { implicit c =>
  SQL("SELECT value from table1 WHERE externalId = 333")
    .map { case Row(value: Option[String]) => value }
} match {
  case Some(x) #:: xs =>
    //????????????
    ("existsWithValue", ???)
  case None #::xs => ("existsButNull", Nil)
  case Stream.empty => ("notExists", Nil)
}

请注意，如果值存在且不为NULL，并且记录多于1条记录，则value不能为NULL.例如，这种情况不可能

Note that if value exists and is not NULL and if there are more than 1 record of it then value can't be NULL. For example, this situation is NOT possible

1 123 "Value1"
2 123 "Value2"
3 123  NULL    -- NOT possible  
4 123 "Value4"

这与Stream + Option的模式匹配有关，但是我不知道该怎么做.

It has to do with pattern matching of Stream + Option but I can't figure out how to do this.

推荐答案

在流上进行模式匹配可能不是一个好主意，我假设要返回的元素数量足够小以适合内存吗?仅在元素可能不适合内存的情况下才使用流，否则将流转换为列表然后再使用列表会更容易:

Pattern matching on a stream is probably not a good idea, I'm assuming the number of elements being returned is small enough to fit into memory? You would only work with a stream if it's likely that the elements won't fit in memory, otherwise it's much easier to convert the stream to a list, and then work with the list:

DB.withConnection { implicit c =>
  SQL("SELECT value from table1 WHERE externalId = 333")
    .map { case Row(value: Option[String]) => value }
    .toList
} match {
  case Nil => ("notExists", Nil)
  case List(None) => ("existsButNull", Nil)
  case xs => ("existsWithValue", xs.flatMap(_.toList))
}

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