Question

我有这个片段描述Notification和Notified个实体：

Notification
  type          NotiType
  release       ReleaseId
  date          UTCTime
Notified
  aboutWhat     NotificationId
  unread        Bool
  user          UserId

现在我要写这个：

-- | Mark specified notification as already seen by specific user. Note that
-- we use 'ReleaseId' to select notification, so this may result in several
-- notifications marked as “read” if they happen to be about the same
-- release. This is generally what you want.

markAsRead
  :: ReleaseId         -- ^ Release in question
  -> UserId            -- ^ User who has seen mentioned release
  -> SqlPersistM ()
markAsRead release user = do
  ns <- selectKeysList [ NotificationRelease ==. release ] []
  updateWhere [ NotifiedAboutWhat <-. ns
              , NotifiedUnread    ==. True
              , NotifiedUser      ==. user ]
              [ NotifiedUnread    =.  False ]

这样可行，但是将通知列表提取为 list ，然后使用它来选择另一个表中的内容......这样做并不完全正确。显然我需要在这里加入，然后我将能够有效地更新所有内容。

如何在纯persistent中执行此操作？是否有可能在这种情况下与persistent保持这种任务是一个好主意？我应该使用esqueleto吗？看起来我需要学习不同的DSL才能使用它，所以我不确定是否要切换。

如何使用markAsRead正确编写persistent（如果可能）？

Answer 1

是Esqueleto，如果你想加入。如果您的数据库和数据建模支持，那么持久性可以很好地与嵌入数据配合使用。

Answer 2

正如格雷格所说，Esqueleto是要走的路。您可以尝试阅读its main module documentation。

目前Esqueleto不支持UPDATE上的联接。但是，您可以使用子查询获得相同的效果。

未经测试的代码可以帮助您入门：

-- | Mark specified notification as already seen by specific user. Note that
-- we use 'ReleaseId' to select notification, so this may result in several
-- notifications marked as “read” if they happen to be about the same
-- release. This is generally what you want.
markAsRead
  :: ReleaseId         -- ^ Release in question
  -> UserId            -- ^ User who has seen mentioned release
  -> SqlPersistM ()
markAsRead release user = 
  update $ \n -> do
  set n [ NotifiedUnread =. val False ]
  where_ $
    n ^. NotifiedUnread  ==. val True &&.
    n ^. NotifiedUser    ==. val user &&.
    n ^. NotifiedAboutWhat `in_` 
      (subList_select $
       from $ \t -> do
       where_ $ t ^. NotificationRelease ==. val release
       return $ t ^. NotificationId)

内部加入持久性还是应该使用esqueleto？

2 个答案: