（编辑：使用触发器查看下面的解决方案）
问题：默认值不会覆盖将列显式设置为 NULL
根据SQLite docs：
如果用户在执行 INSERT 时没有明确提供值，则 DEFAULT 子句指定用于列的默认值。
问题在于，当 Persistent 进行插入时，它将 createdAt 和 updatedAt 列指定为 NULL：
[Debug#SQL] INSERT INTO "user"("email","created_at","updated_at") VALUES(?,?,?); [PersistText "saurabhnanda@gmail.com",PersistNull,PersistNull]
为了得出这个结论，我修改了你的 sn-p 以添加 SQL 日志记录（我只是复制了runSqlite 的源并将其更改为日志到 STDOUT）。我使用文件而不是内存数据库，这样我就可以在 SQLite 编辑器中打开数据库并验证是否设置了默认值。
-- Pragmas and imports are taken from a snippet in the Yesod book. Some of them may be superfluous.
{-# LANGUAGE EmptyDataDecls #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies #-}
import Database.Persist
import Database.Persist.Sqlite
import Database.Persist.TH
import Control.Monad.IO.Class (liftIO)
import Data.Time
import Control.Monad.Trans.Resource
import Control.Monad.Logger
import Control.Monad.IO.Class
import Data.Text
share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persistLowerCase|
User
email String
createdAt UTCTime Maybe default=CURRENT_TIME
updatedAt UTCTime Maybe default=CURRENT_TIME
deriving Show
|]
runSqlite2 :: (MonadBaseControl IO m, MonadIO m)
=> Text -- ^ connection string
-> SqlPersistT (LoggingT (ResourceT m)) a -- ^ database action
-> m a
runSqlite2 connstr = runResourceT
. runStdoutLoggingT
. withSqliteConn connstr
. runSqlConn
main = runSqlite2 "bar.db" $ do
runMigration migrateAll
userId <- insert $ User "saurabhnanda@gmail.com" Nothing Nothing
liftIO $ print userId
user <- get userId
case user of
Nothing -> liftIO $ putStrLn ("coulnt find userId=" ++ (show userId))
Just u -> liftIO $ putStrLn ("user=" ++ (show user))
这是我得到的输出：
Max@maximilians-mbp /tmp> stack runghc sqlite.hs
Run from outside a project, using implicit global project config
Using resolver: lts-3.10 from implicit global project's config file: /Users/Max/.stack/global/stack.yaml
Migrating: CREATE TABLE "user"("id" INTEGER PRIMARY KEY,"email" VARCHAR NOT NULL,"created_at" TIMESTAMP NULL DEFAULT CURRENT_TIME,"updated_at" TIMESTAMP NULL DEFAULT CURRENT_TIME)
[Debug#SQL] CREATE TABLE "user"("id" INTEGER PRIMARY KEY,"email" VARCHAR NOT NULL,"created_at" TIMESTAMP NULL DEFAULT CURRENT_TIME,"updated_at" TIMESTAMP NULL DEFAULT CURRENT_TIME); []
[Debug#SQL] INSERT INTO "user"("email","created_at","updated_at") VALUES(?,?,?); [PersistText "saurabhnanda@gmail.com",PersistNull,PersistNull]
[Debug#SQL] SELECT "id" FROM "user" WHERE _ROWID_=last_insert_rowid(); []
UserKey {unUserKey = SqlBackendKey {unSqlBackendKey = 1}}
[Debug#SQL] SELECT "email","created_at","updated_at" FROM "user" WHERE "id"=? ; [PersistInt64 1]
user=Just (User {userEmail = "saurabhnanda@gmail.com", userCreatedAt = Nothing, userUpdatedAt = Nothing})
编辑：使用触发器的解决方案：
您可以使用触发器实现 created_at 和 updated_at 列。这种方法有一些很好的优点。基本上，updated_at 无论如何都无法通过 DEFAULT 值强制执行，因此如果您希望数据库（而不是应用程序）对其进行管理，则需要一个触发器。触发器还解决了在执行原始 SQL 查询或批量更新时设置 updated_at 的问题。下面是这个解决方案的样子：
CREATE TRIGGER set_created_and_updated_at AFTER INSERT ON user
BEGIN
UPDATE user SET created_at=CURRENT_TIMESTAMP, updated_at=CURRENT_TIMESTAMP WHERE user.id = NEW.id;
END
CREATE TRIGGER set_updated_at AFTER UPDATE ON user
BEGIN
UPDATE user SET updated_at=CURRENT_TIMESTAMP WHERE user.id = NEW.id;
END
现在输出如预期：
[Debug#SQL] INSERT INTO "user"("email","created_at","updated_at") VALUES(?,?,?); [PersistText "saurabhnanda@gmail.com",PersistNull,PersistNull]
[Debug#SQL] SELECT "id" FROM "user" WHERE _ROWID_=last_insert_rowid(); []
UserKey {unUserKey = SqlBackendKey {unSqlBackendKey = 1}}
[Debug#SQL] SELECT "email","created_at","updated_at" FROM "user" WHERE "id"=? ; [PersistInt64 1]
user=Just (User {userEmail = "saurabhnanda@gmail.com", userCreatedAt = Just 2016-02-12 16:41:43 UTC, userUpdatedAt = Just 2016-02-12 16:41:43 UTC})
触发器解决方案的主要缺点是设置触发器有点麻烦。
编辑 2：避免 Maybe 和 Postgres 支持
如果您想避免为createdAt 和updatedAt 设置Maybe 值，您可以在服务器上将它们设置为一些虚拟值，如下所示：
-- | Use 'zeroTime' to get a 'UTCTime' without doing any IO.
-- The main use case of this is providing a dummy-value for createdAt and updatedAt fields on our models. Those values are set by database triggers anyway.
zeroTime :: UTCTime
zeroTime = UTCTime (fromGregorian 1 0 0) (secondsToDiffTime 0)
然后让服务器通过触发器设置值。有点hacky，但在实践中效果很好。
Postgresql 触发器
OP 要求使用 SQLite，但我确信人们也在为其他数据库阅读此内容。这是 Postgresql 版本：
CREATE OR REPLACE FUNCTION create_timestamps()
RETURNS TRIGGER AS $$
BEGIN
NEW.created_at = now();
NEW.updated_at = now();
RETURN NEW;
END;
$$ language 'plpgsql';
CREATE OR REPLACE FUNCTION update_timestamps()
RETURNS TRIGGER AS $$
BEGIN
NEW.updated_at = now();
RETURN NEW;
END;
$$ language 'plpgsql';
CREATE TRIGGER users_insert BEFORE INSERT ON users FOR EACH ROW EXECUTE PROCEDURE create_timestamps();
CREATE TRIGGER users_update BEFORE UPDATE ON users FOR EACH ROW EXECUTE PROCEDURE update_timestamps();