假设您有两个模型(Foo 和 Bar)并且它们都相互引用(Foo 有 barRef 类型为 BarId 和 Bar 有 fooRef 类型为 FooId)。一切顺利:
#!/usr/bin/env stack
{- stack script --resolver=lts-9.21 --package=persistent-template -}
{-# LANGUAGE GADTs, GeneralizedNewtypeDeriving, QuasiQuotes, TemplateHaskell #-}
{-# LANGUAGE TypeFamilies, MultiParamTypeClasses #-}
import Database.Persist.TH
mkPersist sqlSettings [persistLowerCase|
Foo
barModel BarId
fooField Int
Bar
fooModel FooId
barField Int
|]
main = pure ()
但是如果我将引用类型包装为Maybe(只是一个例子,它可以是Vector 或其他任何类型):
#!/usr/bin/env stack
{- stack script --resolver=lts-9.21 --package=persistent-template -}
{-# LANGUAGE GADTs, GeneralizedNewtypeDeriving, QuasiQuotes, TemplateHaskell #-}
{-# LANGUAGE TypeFamilies, MultiParamTypeClasses #-}
import Database.Persist.TH
mkPersist sqlSettings [persistLowerCase|
Foo
barModel (Maybe BarId)
fooField Int
Bar
fooModel FooId
barField Int
|]
main = pure ()
失败并出现此错误:
Not in scope: type constructor or class ‘BarId’
我应该如何解决这个问题?
最佳答案
您用于 Bar
模型的语法不正确。这样的事情应该有效:
#!/usr/bin/env stack
-- stack script --resolver lts-12.7
{-# LANGUAGE GADTs, GeneralizedNewtypeDeriving, QuasiQuotes, TemplateHaskell #-}
{-# LANGUAGE TypeFamilies, MultiParamTypeClasses #-}
import Database.Persist.TH
mkPersist sqlSettings [persistLowerCase|
Foo
barModel BarId Maybe
fooField Int
Bar
fooModel FooId
barField Int
|]
main = pure ()
参见 official docs供引用。
关于database - haskell "persistent"模型 : How to correctly define cross-reference?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/52247307/