如何先从一个流中获取然后传递到另一个流

Question

我需要调用 cassandra 来获取日期，然后将获取的日期传递给另一个将数据插入数据库的流。

def fetchDate: Future[Done] =
  readJournal(persistenceKey)
    .drop(3)
    .take(1)
    .map(l => l.mydate)
    .runWith(Sink.ignore)

def insertRowsToDb: Future[Done] = 
  readJournal(somePersistenceKey)
    .drop(4)
    .take(1)
    .map(data => MyClass(data))
    .mapAsync(1) { myData => 
      for {
        insert <- myRepository.insert(data.id, fetchDate) //error here because fetchDate is unavailable 
      }
    }

  class MyRepository(tableName: String) {
    def insert(id: String, fetchedDate: Long): Future[Int] =
      config.db.run {
        sqlu"""INSERT INTO #${tableName}
           VALUES (
            ${id},
            ${fetchedDate}
           )
          """
      }

问题

• 如何先执行fetchDate，然后将其结果传递给myRepository.insert 行？

Answer 1

你有两个问题。首先是fetchDate 甚至没有日期！解决这个问题意味着使用Sink 运行，而不仅仅是“忽略”：

def fetchDateFut: Future[Date] =
  readJournal(persistenceKey)
    .drop(3)
    .take(1)
    .map(l => l.mydate)
    .runWith(Sink.last)

然后，您需要flatMap 您的Future 将该日期纳入范围：

def insertRowsToDb: Future[Done] = fetchDateFut.flatMap { fetchDate: Date =>
  readJournal(somePersistenceKey)
    .drop(4)
    .take(1)
    .map(data => MyClass(data))
    .mapAsync(1) { myData => 
      for {
        insert <- myRepository.insert(data.id, fetchDate)
      }
    }
}