Question

所以我计划将Persistent 0.9.0.1与Sqlite一起使用，我正在阅读教程：http://www.yesodweb.com/book/persistent

带插入和查询的简单代码段无法编译：

-- START
{-# LANGUAGE QuasiQuotes, TypeFamilies, GeneralizedNewtypeDeriving, TemplateHaskell,
             OverloadedStrings, GADTs, FlexibleContexts #-}
import Database.Persist
import Database.Persist.TH
import Database.Persist.Sqlite
import Control.Monad.IO.Class (liftIO)

share [mkPersist sqlSettings, mkSave "entityDefs"] [persist|
Person
    name String
    age Int
|]

main = withSqliteConn ":memory:" $ runSqlConn $ do
    runMigration $ migrate entityDefs (undefined :: Person) -- this line added: that's it!
    michaelId <- insert $ Person "Michael" 26
    michael <- get michaelId
    liftIO $ print michael
-- STOP

转储拼接显示生成的Person没有承诺的Eq，Show个实例，奇怪：

data PersonGeneric (backend :: (* -> *) -> * -> *)
    = Person {personName :: String, personAge :: Int}
type Person =
    PersonGeneric Database.Persist.GenericSql.Raw.SqlPersist

我这样破解：

    liftIO $ putStrLn $ "name: " ++ (personName $ fromJust michael) ++ ", age: " ++ (show $ personAge $ fromJust michael)

然后插入无法从Sqlite返回主键：

Migrating: CREATE TABLE "person"("id" INTEGER PRIMARY KEY,"name" VARCHAR NOT NULL,"age" INTEGER NOT NULL)
proto: Pattern match failure in do expression at Database/Persist/GenericSql.hs:109:25-45

有什么想法吗？ Persistent是否适用于Sqlite？

Answer 1

从版本0.8更改为0.9，默认情况下我们不再包含Eq和Show个实例。要添加它们，请在age Int下面添加以下行：

deriving Show Eq

这本书仍然针对Yesod版本0.10，但我会在接下来的几天内尝试更新它。

持久性的新手问题

1 个答案: