使用Aeson持久地将UTCTime解析为记录

Question

我有我的模特models.persistentmodels

...

Thing
    title Text
    price Int 
    kosher Bool
    optionalstuff [Text] Maybe
    createdat UTCTime
    updatedat UTCTime
    deriving Show

...

它包含两个时间字段，它们是UTCTime。

我通过AJAX收到JSON中的[[几乎 a Thing]。但是用户JSON不应具有createdat和updatedat或kosher。因此，我们需要填写它们。

postNewEventR = do
    inputjson <- requireCheckJsonBody :: Handler Value
...
    -- get rawstringofthings from inputjson
...

    let objectsMissingSomeFields = case (decode (BL.fromStrict $ TE.encodeUtf8 rawstringofthings) :: Maybe [Object]) of
                        Nothing -> error "Failed to get a list of raw objects."
                        Just x  -> x

    now <- liftIO getCurrentTime

    -- Solution needs to go here:
    let objectsWithAllFields = objectsMissingSomeFields

    -- We hope to be done
    let things = case (eitherDecode $ encode objectsWithAllFields) :: Either String [Thing] of
                        Left err -> error $ "Failed to get things because: " <> err
                        Right xs  -> xs

错误“无法获取东西”出现在这里，因为我们解析的JSON对象缺少模型中所需的字段。

Answer 1

但是UTCTime的方式是read与aeson's way to parse UTCTime之间存在一些奇怪的差异。因此，当我将UTCTime打印到Aeson String中时，我需要将UTCTime打印为以后需要的格式：

tackOnNeccessaryThingFields :: UTCTime -> Bool -> Object -> Object
tackOnNeccessaryThingFields t b hm = G.fromList $ (G.toList hm) <> [
                                                                              ("createdat", String (pack $ formatTime defaultTimeLocale "%FT%T%QZ" t)),
                                                                              ("updatedat", String (pack $ formatTime defaultTimeLocale "%FT%T%QZ" t)),
                                                                              ("kosher", Bool b)
                                                                 ]

tackOnNeccessaryThingFields _ _ _ = error "This isn't an object."

此修复程序之后，对象具有进行记录所需的所有字段，因此代码给出[Thing]。并且代码运行时也不会出现运行时错误，而不是未能将tshow t解析为UTCTime。
注意：这个关于这个问题的eson github问题似乎已关闭，但似乎不再允许：https://github.com/bos/aeson/issues/197

感谢Artyom：https://artyom.me/aeson#records-and-json-generics

感谢Pbrisbin：https://pbrisbin.com/posts/writing_json_apis_with_yesod/
感谢Snoyman：对于一切