左加入奥帕利

Question

我一直试图在项目中使用Opaleye运行左联接，但无法编译代码。我从两个"模型"开始，它们表示相关联的表：

第一：

data ModelA' a b = Model { primA :: a, foreignA :: b }
type ModelA = ModelA' UUID UUID
type ModelAColumn = ModelA' (Column PGUuid) (Column (Nullable PGUuid))
$(makeAdaptorAndInstance "pModelA" ''ModelA')
table :: Table ModelAColumn ModelAColumn
table = Opaleye.table "model_a" $ pModelA (ModelA (tableColumn "uuid") (tableColumn "foreign"))

还有：

data ModelB' a b = Model { primB :: a, valB :: b }
type ModelB = ModelB' UUID String
type ModelBColumn = ModelB' (Column PGUuid) (Column PGText)
$(makeAdaptorAndInstance "pModelB" ''ModelB')
table :: Table ModelBColumn ModelBColumn
table = Opaleye.table "model_b" $ pModelB (ModelB (tableColumn "uuid") (tableColumn "val"))

正如类型所反映的，ModelA不能有任何ModelB关联。

我需要一个查询来获得foreignA==primB上表之间的左联接所给出的(ModelA，Maybe ModelB)对。我本来以为它看起来像：

doJoin :: Connection -> IO [(ModelA, Maybe ModelB)]
doJoin conn = runQuery conn query
where
query :: Query (ModelAColumn, Maybe ModelBColumn)
query = leftJoin (queryTable ModelA.table) (queryTable ModelB.table) ((ma, mb) -> foreignA ma .== primB mb)

但这并不奏效。我还尝试了多种变体，特别是我在查询中替换了类型签名，以明确声明右侧列的可空性：

query :: Query (ModelAColumn, (Column (Nullable PGUuid), Column (Nullable PGText))

但这失败了：

没有Data.Profunctor.Product.Default.Class.Default的实例Opaley.Internal.Join.NullMaker ModelBColumn(Column(NullablePGUuid)，列(可为Null的PGText)。

如何在Opaleye中进行此查询？

Answer 1

这里有几个误解。我在下面制作了一个完整的工作版本。

首先，leftJoin的返回类型不是

Query (ModelAColumn, Maybe ModelBColumn)

你必须做

type ModelBNullableColumn = ModelB' (Column (Nullable PGUuid))
(Column (Nullable PGText))

然后使用

Query (ModelAColumn, ModelBNullableColumn)

其次，runQuery的返回类型不是

IO [(ModelA, Maybe ModelB)]

你必须做

type ModelBMaybe = ModelB' (Maybe UUID) (Maybe String)

并使用

IO [(ModelA, ModelBMaybe)]

这些差异的原因是Nullable和Maybe必须直接应用于每一列，而ModelBColumn和ModelB中的值不能应用于整个值。

(还有一些奇怪的语法错误，比如

ModelA { tableColumn "uuid", tableColumn "foreign" }

这意味着您的代码没有编译的希望。我也修好了。)

{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
import           Opaleye hiding (table)
import qualified Opaleye
import Data.Profunctor.Product.TH
import Database.PostgreSQL.Simple hiding (Query)
import Data.UUID
data ModelA' a b = ModelA { primA :: a, foreignA :: b }
type ModelA = ModelA' UUID (Maybe UUID)
type ModelAColumn = ModelA' (Column PGUuid) (Column (Nullable PGUuid))
$(makeAdaptorAndInstance "pModelA" ''ModelA')
modelAtable :: Table ModelAColumn ModelAColumn
modelAtable = Opaleye.table "model_a" $ pModelA ModelA { primA = tableColumn "uuid", foreignA = tableColumn "foreign" }
data ModelB' a b = ModelB { primB :: a, valB :: b }
type ModelB = ModelB' UUID String
type ModelBMaybe = ModelB' (Maybe UUID) (Maybe String)
type ModelBColumn = ModelB' (Column PGUuid) (Column PGText)
type ModelBNullableColumn = ModelB' (Column (Nullable PGUuid)) (Column (Nullable PGText))
$(makeAdaptorAndInstance "pModelB" ''ModelB')
modelBtable :: Table ModelBColumn ModelBColumn
modelBtable = Opaleye.table "model_b" $ pModelB ModelB { primB = tableColumn "uuid", valB = tableColumn "val" }
doJoin :: Connection -> IO [(ModelA, ModelBMaybe)]
doJoin conn = runQuery conn query
where
query :: Query (ModelAColumn, ModelBNullableColumn)
query = leftJoin (queryTable modelAtable) (queryTable modelBtable) ((ma, mb) -> matchNullable (pgBool False) (.== primB mb) (foreignA ma))
main :: IO ()
main = return ()

Answer 2

由于有人提出并回答了这个问题，Opaleye引入了一种更简单的左联接方法。查看名为optional的函数。

Answer 3

尝试更改以下内容：

query :: Query (ModelAColumn, Maybe ModelBColumn)

至

query :: Query ((Column PGUuid) (Column (Nullable PGUuid))
, (Column (Nullable PGUuid)) (Column (Nullable PGText)))

尝试在不调用runQuery的情况下键入check。一旦成功，请返回解决方案的其余部分。

Answer 4

您需要使用Functional Join，特别是leftJoinF。

看看这个https://hackage.haskell.org/package/opaleye-0.5.4.0/docs/Opaleye-FunctionalJoin.html

您需要在两个表上提供select的结果，作为它的第4个和第5个参数。相当于foreignA == primB作为第三个参数。

对于第二个参数，您必须指定一些默认值，只要返回的Maybe ModelB是Nothing，就应该使用该值