如何创建查询数据库的自定义字段？ [英] How to create a custom field which queries the database?

问题描述

我是Yesod的新手，希望创建一个自定义字段，我需要查询。

我的模型如下：

  Article 
 artname文本
 title文本
正文Text 
 Parent ArticleId也许
 
 UniqueArt artname 
 
类型化
  

我想创建用户输入 artname 而不是数字ID的父字段，但它将成为存储在数据库中的真实ID。

我不能使用 checkMMap ，因为反转函数在IO之外工作。

从我对字段处理的理解中， fieldParse 获取用户输入的值，并尝试将其转换为ArticleId，同时 fieldView 接受ArticleId并显示更人性化的版本。

到目前为止，我发现了以下内容：

  parentField :: Field sub ArticleId 
 parentField = Field 
 {fieldParse = \rawVals _  - > 
 let（name：[]）= rawVals 
 marticle<  -  runDB $ getBy（UniqueArt name）
 case b 
 Nothing  - >返回$（Left。SomeMessage）（Article name invalid。:: Text）
只是文章 - > return $（Right。Just）（entityKey article）
 
，fieldView = \idAttr nameAttr attrs eResult isReq  - > 
 case 
的eResult右键 - > do 
 marticle<  -  runDB $ get key 
 let name = case marticle of 
 Just article  - >右（文章名称文章）
无 - > Left（Article key invalid。:: Text）
 
（fieldView textField）idAttr nameAttr attrs name isReq 
 
 Left _  - > （fieldView textField）idAttr nameAttr attrs eResult isReq 
} 
  

GHC不喜欢 marticle< - runDB $ get key 行，并给我以下错误：

 无法将类型'HandlerT site1 IO'
与'WidgetT（HandlerSite sub）IO'
预期类型：HandlerT site1 IO也许文章）
  - > （也许Article  - > HandlerT site1 IO（））
  - > WidgetT（HandlerSite sub）IO（）
实际类型：HandlerT site1 IO（可能为文章）
  - > （也许Article  - > HandlerT site1 IO（）） - > HandlerT site1 IO（）
相关绑定包括
 parentField :: Field sub ArticleId 
（绑定在Handler / Article.hs：39：1）
在'do 'block：marticle<  -  runDB $ get key 
在表达式中：
 do {marticle<  -  runDB $ get key; 
让name = ...; 
（fieldView textField）idAttr nameAttr attrs name isReq} 
在另一种情况下：
右键
  - >做{marticle<  -  runDB $ get key; 
让name = ...; 
（fieldView textField）idAttr nameAttr attrs name isReq} 
  

任何想法？它是一个 lift 我忘了吗？

解决方案

为了能够在 fieldParse 和 fieldView 中执行查询，我需要进行一些调整：

• parentField 签名必须完全指定。需要设置 YesodPersist 和 YesodPersistBackend 约束，因为 runDB 调用。


• fieldView 中的查询需要转换为 Widget Widget 的函数内部工作。这就是为什么要使用 handlerToWidget 函数。


• 原始代码基于 textField 字段，但是这强加了其他限制。因此，我定义了自己的 whamlet 。


• 缺少 fieldEnctype 。

以下是更新过的源代码：

  parentField :: YesodPersist网站
 => YesodPersistBackend网站〜SqlBackend 
 => RenderMessage（HandlerSite（HandlerT site IO））FormMessage 
 => Field（HandlerT site IO）ArticleId 
 parentField = Field 
 {fieldParse = \rawVals _  - > （名称：[]）= rawVals 
 articleM<  -  runDB $ getBy（UniqueArt name）
 return $ case articleM of 
 Nothing  - > （Left。SomeMessage）（Article name invalid。:: Text）
只是文章 - > （Right。Just）（entityKey article）
 
，fieldView = \ name name attrs resultE isReq  - > 
案例resultE of 
右键 - >做
 articleM<  -  handlerToWidget。 runDB $ get key 
 let value = case article of 
 Just article  - >右（文章名称文章）
无 - > Left（Article key invalid。:: Text）
 
 parentHtml ident name attrs value isReq 
 
 Left err  - > parentHtml ident name attrs（Left err）isReq 
，fieldEnctype = UrlEncoded 
} 
 where parentHtml ident name attrs val isReq = 
 [whamlet | $ newline never 
<输入id =＃{ident}
 name =＃{name}
 * {attrs} 
 type =text
：isReq：required 
 value =＃{or id id val}> 
 |] 
  

I’m new to Yesod and would like to create a custom field in which I need to do a query.

My model is the following:

Article
    artname     Text
    title       Text
    body        Text
    parent      ArticleId Maybe

    UniqueArt   artname

    deriving    Typeable

I want to create a "parent field" in which the user enters an artname instead of a numerical id, but it will be the real id which will be stored in the database.

I cannot use checkMMap since the invert function works outside of IO.

From what I understood of the field processing, fieldParse takes the value entered by the user and tries to convert it to an ArticleId while fieldView takes an ArticleId and shows a more human version.

What I’ve come up until now is the following:

parentField :: Field sub ArticleId
parentField = Field
    { fieldParse = \rawVals _ -> do
            let (name:[]) = rawVals
            marticle <- runDB $ getBy (UniqueArt name)
            case marticle of
                Nothing      -> return $ (Left . SomeMessage) ("Article name invalid." :: Text)
                Just article -> return $ (Right . Just) (entityKey article)

    , fieldView = \idAttr nameAttr attrs eResult isReq ->
            case eResult of
                Right key -> do
                    marticle <- runDB $ get key
                    let name = case marticle of
                                   Just article -> Right (articleArtname article)
                                   Nothing      -> Left ("Article key invalid." :: Text)

                    (fieldView textField) idAttr nameAttr attrs name isReq

                Left _ -> (fieldView textField) idAttr nameAttr attrs eResult isReq
    }

GHC doesn’t like the marticle <- runDB $ get key line and gives me the following error:

Handler/Article.hs:50:21:
    Couldn't match type ‘HandlerT site1 IO’
                  with ‘WidgetT (HandlerSite sub) IO’
    Expected type: HandlerT site1 IO (Maybe Article)
                   -> (Maybe Article -> HandlerT site1 IO ())
                   -> WidgetT (HandlerSite sub) IO ()
      Actual type: HandlerT site1 IO (Maybe Article)
                   -> (Maybe Article -> HandlerT site1 IO ()) -> HandlerT site1 IO ()
    Relevant bindings include
      parentField :: Field sub ArticleId
        (bound at Handler/Article.hs:39:1)
    In a stmt of a 'do' block: marticle <- runDB $ get key
    In the expression:
      do { marticle <- runDB $ get key;
           let name = ...;
           (fieldView textField) idAttr nameAttr attrs name isReq }
    In a case alternative:
        Right key
          -> do { marticle <- runDB $ get key;
                  let name = ...;
                  (fieldView textField) idAttr nameAttr attrs name isReq }

Any idea ? Is it a lift I forgot ?

解决方案

In order to be able to do queries inside fieldParse and fieldView, I needed some adjustments:

• the parentField signature must be fully specified. The YesodPersist and YesodPersistBackend constraints needs to be set because of the runDB calls.
• the query inside fieldView needs to be translated to a Widget because it is working inside a function that outputs a Widget. That’s why the handlerToWidget function is used.
• the original code was based on the textField field but this imposed other constraints. Therefore I defined my own whamlet.
• the fieldEnctype was missing.

Here is the updated source code:

parentField :: YesodPersist site
            => YesodPersistBackend site ~ SqlBackend
            => RenderMessage (HandlerSite (HandlerT site IO)) FormMessage
            => Field (HandlerT site IO) ArticleId
parentField = Field
    { fieldParse = \rawVals _ -> do
            let (name:[]) = rawVals
            articleM <- runDB $ getBy (UniqueArt name)
            return $ case articleM of
                Nothing      -> (Left . SomeMessage) ("Article name invalid." :: Text)
                Just article -> (Right . Just) (entityKey article)

    , fieldView = \ident name attrs resultE isReq ->
            case resultE of
                Right key -> do
                    articleM <- handlerToWidget . runDB $ get key
                    let value = case articleM of
                                   Just article -> Right (articleArtname article)
                                   Nothing      -> Left ("Article key invalid." :: Text)

                    parentHtml ident name attrs value isReq

                Left err -> parentHtml ident name attrs (Left err) isReq
    , fieldEnctype = UrlEncoded
    }
    where parentHtml ident name attrs val isReq =
            [whamlet|$newline never
                <input id="#{ident}"
                       name="#{name}"
                       *{attrs}
                       type="text"
                       :isReq:required
                       value="#{either id id val}">
            |]

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