Question

我有一个实体定义，其中包含UTCTime类型的字段。

Foo
    time UTCTime
    ...

我想选择一系列按时间排序的实体。如果两个实体有相同的时间，我不在乎哪一个是第一个，我只想让订单每次都相同。实体键是唯一的，它似乎按键排序将是完美的选择。

是否可以执行selectList之类的查询并按主键排序匹配的行？可替代地

是否可以在实体中包含主键？
或者，还有其他方法可以确定行的顺序吗？

当然，我总是可以按时间要求订购，然后自己按键订购退回的列表。但这排除了使用SelectOp s OffsetBy和LimitTo，因为我事先无法知道有多少实体会有相同的时间。

Answer 1

我不确定SelectOpt仅适用于实体值，所以我在一小段代码中检查了它。假设一个实体看起来像这样：

EntityA
    key Text
    time UTCTime
    deriving Show

您可以列出您感兴趣的SelectOpt并获得理想的输出。

示例代码如下所示：

main :: IO ()
main = runSqlite ":memory:" $ do
    runMigration migrateAll

    currentTime <- liftIO getCurrentTime

    -- insert some data
    _ <- insert $ EntityA "first" currentTime
    _ <- insert $ EntityA "second" currentTime
    _ <- insert $ EntityA "third" currentTime
    _ <- insert $ EntityA "fourth" currentTime

    currentTime <- liftIO getCurrentTime

    -- insert more data
    _ <- insert $ EntityA "fifth" currentTime
    _ <- insert $ EntityA "sixth" currentTime
    _ <- insert $ EntityA "seventh" currentTime
    _ <- insert $ EntityA "eighth" currentTime

    currentTime <- liftIO getCurrentTime

    -- insert more data
    _ <- insert $ EntityA "this one will be first" currentTime

    sorted <- selectList [] [Desc EntityATime, Asc EntityAId]
    mapM_ (liftIO . print) sorted

，输出为：

Migrating: CREATE TABLE "entity_a"("id" INTEGER PRIMARY KEY,"key" VARCHAR NOT NULL,"time" TIMESTAMP NOT NULL)
Entity {entityKey = Key {unKey = PersistInt64 9}, entityVal = EntityA {entityAKey = "this one will be first", entityATime = 2014-05-07 20:15:35.627561 UTC}}
Entity {entityKey = Key {unKey = PersistInt64 5}, entityVal = EntityA {entityAKey = "fifth", entityATime = 2014-05-07 20:15:35.627026 UTC}}
Entity {entityKey = Key {unKey = PersistInt64 6}, entityVal = EntityA {entityAKey = "sixth", entityATime = 2014-05-07 20:15:35.627026 UTC}}
Entity {entityKey = Key {unKey = PersistInt64 7}, entityVal = EntityA {entityAKey = "seventh", entityATime = 2014-05-07 20:15:35.627026 UTC}}
Entity {entityKey = Key {unKey = PersistInt64 8}, entityVal = EntityA {entityAKey = "eighth", entityATime = 2014-05-07 20:15:35.627026 UTC}}
Entity {entityKey = Key {unKey = PersistInt64 1}, entityVal = EntityA {entityAKey = "first", entityATime = 2014-05-07 20:15:35.626622 UTC}}
Entity {entityKey = Key {unKey = PersistInt64 2}, entityVal = EntityA {entityAKey = "second", entityATime = 2014-05-07 20:15:35.626622 UTC}}
Entity {entityKey = Key {unKey = PersistInt64 3}, entityVal = EntityA {entityAKey = "third", entityATime = 2014-05-07 20:15:35.626622 UTC}}
Entity {entityKey = Key {unKey = PersistInt64 4}, entityVal = EntityA {entityAKey = "fourth", entityATime = 2014-05-07 20:15:35.626622 UTC}}

如果您感兴趣，我可以发布示例项目（它在fpcomplete.com上），但我想这很好地说明了它。

是否可以按主键订购查询？

1 个答案: