在Maybe Relation上运行join

Question

我有一个模型

Assignment
   blah Text
   ....

和模型

File
    assignmentId AssignmentId Maybe
    ...

，我想在连接查询中获得与赋值相关的所有文件。我已经尝试了Esqueleto和runJoin与selectOneMany，但没有任何运气，所以我考虑不使用连接，或使用rawSql。这看起来真的不是个好主意，但我想不明白。是否支持该功能?

Answer 1

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