Yesod /持久性一对一查询 [英] Yesod/Persistent one-to-one query

问题描述

在Yesod / Persistent中，我有模型设置如下所示：

  User 
 ident Text 
密码文本也许
 UniqueUser ident 
问题
 title文本
 asker UserId Eq 
  

我列出了问题 s，并希望检索 User 秒。我会如何去做这件事？

我想过加入，但这些是一对多的，而不是一对一的（我想它不会没关系，但我想要一个更简单的解决方案）。手动进行连接也是一种选择，但我担心性能 - 我有

  questions<  -  runDB $ selectList [QuestionTitle！=。 ] [LimitTo 10] 
 let askerIds = map（\（Entity _q） - > questionAsker q）questions 
 askers<  -  sequence $ map（runDB。get）askerIds 
让questionsAndAskers = zip问题提问者
  

但我担心使用 runDB在 map 中（它不会为每个用户向数据库发出一个单独的请求吗？）

有没有更好的/更习惯的方式来达到这个目的？ 但是我会将整个内容粘贴在 runDB 中：

  runDB $ selectList [QuestionTitle！=。 ]] [LimitTo 10]>> = mapM（\qe @（Entity _q） - > do 
 asker<  -  get $ questionAsker q 
 return（qe，asker）） 
  

Say, in Yesod/Persistent, I have models setup like so:

User
    ident Text
    password Text Maybe
    UniqueUser ident
Question
    title Text
    asker UserId Eq

And I have a list of Questions, and would like to retrieve the corresponding list of Users. How would I go about doing this?

I've thought about joins, but those are one-to-many, not one-to-one (I suppose it doesn't matter, but I'd like a simpler solution). Manually doing the join is also an option, but I'm worried about performance - I have

questions <- runDB $ selectList [QuestionTitle !=. ""] [LimitTo 10]
let askerIds = map (\(Entity _ q) -> questionAsker q) questions
askers <- sequence $ map (runDB . get) askerIds
let questionsAndAskers = zip questions askers

but I'm worried about using runDB in the map (wouldn't it make a separate request to the database for each user?)

Is there a better/more idiomatic way to achieve this?

解决方案

I haven't type-checked this yet, but I would stick the whole thing inside of runDB:

runDB $ selectList [QuestionTitle !=. ""] [LimitTo 10] >>= mapM (\qe@(Entity _ q) -> do
    asker <- get $ questionAsker q
    return (qe, asker))

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