Haskell Yesod-在将json内的对象转换为模型之前 [英] Haskell Yesod - extracting a object inside a json before converting it to a model
问题描述
假设我有一个像这样的JSON:
Suppose I have a JSON like this:
{
data: {...}
}
和 {...}
代表我的模型.怎么可能在这种情况下,在Handler中获取我的模型?例如,以下内容显然不起作用:
and {...}
represents a model of mine. How could
get my model in this case in the Handler? For instance, the following will not work obviously:
putMyEntityR :: Handler ()
putMyEntityR = do
(Entity id _) <- (...) -- getting the Key
e <- requireJsonBody :: Handler MyEntity
runDB $ replace id e
sendResponseStatus status204 ("UPDATED" :: Text)
如何读取 JSON
,获取 data
对象,然后对其进行解码?
How can I read the JSON
, take the data
object, and only then decode it?
推荐答案
There was some more discussion of this question on a Github Issue, which I'm adding as an answer here because it's more fleshed out. Here's what we arrive at for the Handler function using the helper functions defined below:
postDataCommentR :: Handler Value
postDataCommentR = do
value <- requireJsonBody' -- Parse request body into Value
commentJson <- requireJsonKey "data" value -- Lookup a key from the Value
comment <- (requireJsonParse commentJson :: Handler Comment) -- Parse the Value into a comment record
insertedComment <- runDB $ insertEntity comment
returnJson insertedComment
这些函数获取请求正文并将其解析为aeson Value
:
These functions take the request body and parse it into an aeson Value
:
import qualified Data.Aeson as J
import qualified Data.Aeson.Parser as JP
import Data.Conduit.Attoparsec (sinkParser)
-- These two functions were written by @FtheBuilder
parseJsonBody' :: (MonadHandler m) => m (J.Result Value)
parseJsonBody' = do
eValue <- rawRequestBody $$ runCatchC (sinkParser JP.value')
return $ case eValue of
Left e -> J.Error $ show e
Right value -> J.Success value
-- | Same as 'parseJsonBody', but return an invalid args response on a parse
-- error.
requireJsonBody' :: (MonadHandler m) => m Value
requireJsonBody' = do
ra <- parseJsonBody'
case ra of
J.Error s -> invalidArgs [pack s]
J.Success a -> return a
这些帮助器函数用于将 Value
解析为一条记录:
These helper functions are used to parse that Value
into a record:
requireJsonParse :: (MonadHandler m, FromJSON a) => Value -> m a
requireJsonParse v = case J.fromJSON v of
J.Error s -> invalidArgs [pack s]
J.Success a -> return a
requireJsonKey :: (MonadHandler m) => Text -> Value -> m Value
requireJsonKey key jObject@(Object hashMap) = case lookup key hashMap of
Nothing -> invalidArgs ["Couldn't find a value when looking up the key " <> key <> " in the object: " <> (pack (show jObject))]
Just v -> return v
requireJsonKey key invalid = invalidArgs ["When looking up the key " <> key <> ", expected an object but got a " ++ (pack (show invalid))]
评论
aeson-lens
我没有使用 aeson-lens
,但是无论有没有代码,它的代码都非常相似,因为我们只深入一键.如果我们更深入地研究JSON,则 aeson-lens
会使事情变得更好.
Commentary
aeson-lens
I didn't use aeson-lens
, but the code is pretty similar with or without it, since we're just going one key deep. aeson-lens
would make things nicer if we were traversing deeper into the JSON.
一旦定义了辅助函数,您仍然可以使用几行来解析 Value
,然后查找 data
键,然后创建记录.您可以采取一些措施来缩短它的长度,但是最终@Carsten建议的包装器的长度应该差不多,而且复杂度也要低得多.
Once you get the helper functions defined, you still have a couple lines to parse a Value
, then lookup the data
key, then create your record. You can do things to make this shorter, but ultimately the wrapper that @Carsten recommended would be of similar length with less complexity, imo.
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