Haskell Esqueleto 3 表连接

Question

这些是我从三个表中进行 SELECT 的尝试。但他们不编译，我不明白错误（我不知道为什么它需要一个元组 (Entity Issue, b0) 而不是我认为代码试图获取的三元组）。

尝试 1：

{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TypeFamilies #-}

module Handler.Support where

import           Import hiding ((==.))
import qualified Database.Esqueleto as E
import           Database.Esqueleto      ((^.), (==.), (&&.))
import           Data.Traversable

getSupportR :: CustomerId -> Handler Html
getSupportR customerId = do
  customer_issues_followUps_list <- runDB $
    E.select $
    E.from $ \(i, f, c) -> do
    E.where_ (i ^. IssueCustomerId ==. E.val customerId &&. i ^. IssueId ==. f ^. FollowUpIssueId &&. i ^. IssueCustomerId ==. c ^. CustomerId)
    return (i, f, c)
  let issues = map listToMaybe . group . sort . fst . unzip $ customer_issues_followUps_list
  defaultLayout $ do
    setTitle "Your Licenses"
    $(widgetFile "support-display")

错误 1：

/home/hhefesto/dev/laurus-nobilis/src/Handler/Support.hs:41:5: error:
    • Couldn't match type ‘(ra, rb, rc)’ with ‘(Entity Issue, b0)’
        arising from a functional dependency between:
          constraint ‘Database.Esqueleto.Internal.Sql.SqlSelect
                        (E.SqlExpr (Entity Issue), E.SqlExpr (Entity FollowUp),
                         E.SqlExpr (Entity Customer))
                        (Entity Issue, b0)’
            arising from a use of ‘E.select’
          instance ‘Database.Esqueleto.Internal.Sql.SqlSelect
                      (a3, b3, c) (ra3, rb3, rc3)’
            at <no location info>
    • In the second argument of ‘($)’, namely
        ‘E.select
           $ E.from
               $ \ (i, f, c)
                   -> do E.where_
                           (i ^. IssueCustomerId ==. E.val customerId
                              &&.
                                i ^. IssueId ==. f ^. FollowUpIssueId
                                  &&. i ^. IssueCustomerId ==. c ^. CustomerId)
                         return (i, f, c)’
      In a stmt of a 'do' block:
        customer_issues_followUps_list <- runDB
                                            $ E.select
                                                $ E.from
                                                    $ \ (i, f, c)
                                                        -> do E.where_
                                                                (i ^. IssueCustomerId
                                                                   ==. E.val customerId
                                                                   &&.
                                                                     i ^. IssueId
                                                                       ==. f ^. FollowUpIssueId
                                                                       &&.
                                                                         i ^. IssueCustomerId
                                                                           ==. c ^. CustomerId)
                                                              return (i, f, c)
      In the expression:
        do customer_issues_followUps_list <- runDB
                                               $ E.select $ E.from $ \ (i, f, c) -> do ...
           let issues
                 = map listToMaybe . group . sort . fst . unzip
                     $ customer_issues_followUps_list
           defaultLayout
             $ do setTitle "Your Licenses"
                  (do ...)
   |
41 |     E.select $
   |     ^^^^^^^^^^...

尝试 2：

{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE TypeFamilies #-}

module Handler.Support where

import           Import hiding ((==.))
import qualified Database.Esqueleto as E
import           Database.Esqueleto      ((^.), (==.), (&&.))
import           Data.Traversable

getSupportR :: CustomerId -> Handler Html
getSupportR customerId = do
  customer_issues_followUps_list <- runDB $
    E.select $
    E.from $ \(i `E.InnerJoin` f `E.InnerJoin` c) -> do
    E.on (c ^. CustomerId ==. i ^. IssueCustomerId)
    E.on (i ^. IssueId ==. f ^. FollowUpIssueId)
    E.where_ (i ^. IssueCustomerId ==. E.val customerId)
    return (i, f, c)
  let issues = map listToMaybe . group . sort . fst . unzip $ customer_issues_followUps_list
  defaultLayout $ do
    setTitle "Your Licenses"
    $(widgetFile "support-display")

错误 2：

/home/hhefesto/dev/laurus-nobilis/src/Handler/Support.hs:40:5: error:
    • Couldn't match type ‘(ra, rb, rc)’ with ‘(Entity Issue, b0)’
        arising from a functional dependency between:
          constraint ‘Database.Esqueleto.Internal.Sql.SqlSelect
                        (E.SqlExpr (Entity Issue), E.SqlExpr (Entity FollowUp),
                         E.SqlExpr (Entity Customer))
                        (Entity Issue, b0)’
            arising from a use of ‘E.select’
          instance ‘Database.Esqueleto.Internal.Sql.SqlSelect
                      (a2, b2, c) (ra2, rb2, rc2)’
            at <no location info>
    • In the second argument of ‘($)’, namely
        ‘E.select
           $ E.from
               $ \ (i `E.InnerJoin` f `E.InnerJoin` c)
                   -> do E.on (c ^. CustomerId ==. i ^. IssueCustomerId)
                         E.on (i ^. IssueId ==. f ^. FollowUpIssueId)
                         ....’
      In a stmt of a 'do' block:
        customer_issues_followUps_list <- runDB
                                            $ E.select
                                                $ E.from
                                                    $ \ (i `E.InnerJoin` f `E.InnerJoin` c)
                                                        -> do E.on
                                                                (c ^. CustomerId
                                                                   ==. i ^. IssueCustomerId)
                                                              E.on
                                                                (i ^. IssueId
                                                                   ==. f ^. FollowUpIssueId)
                                                              ....
      In the expression:
        do customer_issues_followUps_list <- runDB
                                               $ E.select
                                                   $ E.from
                                                       $ \ (i `E.InnerJoin` f `E.InnerJoin` c)
                                                           -> do ...
           let issues
                 = map listToMaybe . group . sort . fst . unzip
                     $ customer_issues_followUps_list
           defaultLayout
             $ do setTitle "Your Licenses"
                  (do ...)
   |
40 |     E.select $
   |     ^^^^^^^^^^...

这是我的持久模型：

Customer
    email Text
    password Text
    firstName Text
    lastName Text
    address1 Text
    address2 Text
    city Text
    state Text
    zipCode Text
    country Text
    phone Text
    organization Text
    UniqueCustomer email
    deriving Typeable
    deriving Show
    deriving Eq
    deriving Ord
License
    licenseAlias Text
    expirationDate UTCTime
    assignedTo CustomerId
    customerId CustomerId
    deriving Show
    deriving Eq
    deriving Ord
Issue
    customerId CustomerId
    issueSummary Text
    issueDetails Text
    issueState Int
    issueDate UTCTime
    deriving Show
    deriving Eq
    deriving Ord
FollowUp
    issueId IssueId
    followUpDate UTCTime
    followUpAuthor CustomerId
    followUpText Text
    deriving Show
    deriving Eq
    deriving Ord


-- Soon to be deleted:
Email
    email Text
    customerId CustomerId Maybe
    verkey Text Maybe
    UniqueEmail email
Comment json -- Adding "json" causes ToJSON and FromJSON instances to be derived.
    message Text
    customerId CustomerId Maybe
    deriving Eq
    deriving Show

如您所见，两次尝试的错误是相同的：它需要一个元组而不是一个三元组。

任何帮助将不胜感激:)

Answer 1

罪魁祸首在let 声明中：

let issues = map listToMaybe . group . sort . fst . unzip $ customer_issues_followUps_list

fst 和 unzip 使用元组，因此编译器暗示 customer_issues_followUps_list 是一个元组。

要解决这个问题，只需将“-extra”添加到您的 package.yml 依赖项（或您的 cabal 文件）中，并在 let 语句中将 fst 和 unzip 替换为 fst3 和 unzip3，如下所示：

let issues = map listToMaybe . group . sort . fst3 . unzip3 $ customer_issues_followUps_list