如何基於Haskell Groundhog的數據庫類型指定不同的列類型？

Question

在Haskell中使用Groundhog庫時，我期待實現一個在後端是Postgresql時使用「uuid」的列類型，否則只需將「varchar」用於任何其他後端。根據Groundhog.Core中的評論，這應該是可能的，但我不完全確定如何從proxy db解開dbType，並且在groundhog示例中沒有這樣的例子，因爲在這些示例中已經對列類型進行了硬編碼。

我想在Postgresql的大小寫匹配方面提供一些幫助，一旦排序完成，我將解決其餘問題。下面是我在哪裏：

instance PrimitivePersistField UUID where 
    toPrimitivePersistValue _ uuid = PersistString $ show uuid 
    fromPrimitivePersistValue _ (PersistString a) = fromJust $ UUIDmethods.fromString a 

instance PersistField UUID where 
    persistName _ = "UUID" 
    toPersistValues = primToPersistValue 
    fromPersistValues = primFromPersistValue 
    dbType db _ = case db of 
    Postgresql _ -> DbTypePrimitive (DbOther $ OtherTypeDef [Left "uuid"]) False Nothing Nothing 
    _ -> DbTypePrimitive (DbOther $ OtherTypeDef [Left "varchar"]) False Nothing Nothing 

而且在編譯此出現：

Couldn't match expected type ‘proxy db’ 
       with actual type ‘Postgresql’ 
    Relevant bindings include 
     db :: proxy db (bound at basicGroundhog.hs:34:10) 
     dbType :: proxy db -> UUID -> DbType 
     (bound at basicGroundhag.hs:34:3) 
    In the pattern: Postgresql _ 
    In a case alternative: 
     Postgresql _ 
      -> DbTypePrimitive 
       (DbOther $ OtherTypeDef [Left "uuid"]) False Nothing Nothing 
    In the expression: 
     case db of { 
     Postgresql _ 
      -> DbTypePrimitive 
       (DbOther $ OtherTypeDef [Left "uuid"]) False Nothing Nothing 
     _ -> DbTypePrimitive 
       (DbOther $ OtherTypeDef [Left "varchar"]) False Nothing Nothing } 

Answer 1

您可以在運行時與backendName檢查數據庫。最好將DbString作爲默認情況，以便groundhog可以選擇更合適的類型。沒有最大長度聲明的Varchar在MySQL中無效。

dbType db _ = case backendName db of 
    "postgresql" -> DbTypePrimitive (DbOther $ OtherTypeDef [Left "uuid"]) False Nothing Nothing 
    _ -> DbTypePrimitive DbString False Nothing Nothing