在Maybe关系上运行join | 在Maybe关系上运行join

本文介绍了在Maybe关系上运行join的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！

问题描述

我有一个模型

Assignment
   blah Text
   ....

和模型

File
    assignmentId AssignmentId Maybe
    ...

在连接查询中获取与分配相关联的所有文件。我试过Esqueleto和 runJoin 与 selectOneMany 但没有运气，所以我考虑不使用连接，或使用rawSql。这真的不是一个好主意，但我不能想出这一点。

and I want to get all the files associated with an assignment in a join query. I have tried Esqueleto and runJoin with selectOneMany but haven't had any luck, so I am considering not using a join, or using rawSql. That really doesn't seem like a good idea, but I can't figure this out. Is there any support for that feature?

推荐答案

更新，工作示例：

{-# LANGUAGE PackageImports, OverloadedStrings, ConstraintKinds #-}
module Handler.HTest where

import Import
import "esqueleto" Database.Esqueleto as Esql
import "monad-logger" Control.Monad.Logger (MonadLogger)
import "resourcet" Control.Monad.Trans.Resource (MonadResourceBase)
import qualified Data.List as L

getFilesByAssignment :: (PersistQuery (SqlPersist m), MonadLogger m
                                                  , MonadResourceBase m) =>
                        Text -> SqlPersist m [Entity File]
getFilesByAssignment myAssign = do
  result <- select $
        from $ \(assign `InnerJoin` file) -> do
          on (just (assign ^. AssignmentId)
            Esql.==. file ^. FileAssignmentId)
          where_ (assign ^. AssignmentBlah Esql.==. val myAssign)
          return (assign, file)

  return $ map snd (result :: [(Entity Assignment, Entity File)])

(.$) = flip ($)

getTestR :: Handler RepHtml
getTestR = do
       entFiles <- runDB $ getFilesByAssignment "test"
       defaultLayout $ do
               setTitle "Test page"
               entFiles .$ map (show . unKey . entityKey)
                        .$ L.intercalate ", "
                        .$ toHtml
                        .$ toWidget

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