database - Haskell 存储具有持久定义的组合 ADT

Question

我如何创建类似于此 ADT 的 Yesod Persistent 实体:

import Data.Ratio

data Work = Work { name :: String
                 , payment :: WorkPayment
                 , duration :: WorkDuration }

data WorkPayment = WorkPaymentUndefined
                 | WorkPaymentEstimated { workPaymentEstimatedUnity :: TemporalUnity
                                        , workPaymentEstimatedValue :: Ratio Int }
                 | WorkPaymentTotal { workPaymentTotalValue :: Ratio Int }

data WorkDuration = WorkDurationUndefined
                  | WorkDurationEstimated { workDurationEstimatedUnity :: TemporalUnity
                                          , workDurationEstimatedQuantity :: Int }
                  | WorkDurationPermanent

data TemporalUnity = Hour | Day | Week | Month

长名称可以通过使用单独的模块来避免，但我更喜欢在这个例子中将它们放在一起。

对于简单的枚举，它看起来很简单，但我无法弄清楚在这种情况下表将如何，就像在关系数据库(如 MySQL)中一样

Answer 1

要使用 Persistent 存储 ADT，您只需告诉 Persistent 如何从数据库中序列化/反序列化该值。为此，将其设为 PersistField 的实例(将其转换为基本类型，如 Text 或 ByteString)和 PersistFieldSql(指定要使用的数据库列的类型)。

关于(反)序列化数据，您有多种选择。我想编码/解码为二进制值会更快，但我推荐 JSON，因为:

• 它在数据库级别是人类可读的
• 一些数据库内置了对它的支持
• 它是可移植的
• 通过 to/fromPersistValueJSON 将支持内置到 Persistent 中。

举个例子:

-- Add this extension to derive To/FromJSON instances; see Data.Aeson for details
{-# LANGUAGE DeriveGeneric #-}

module Models.Work where

import Database.Persist.Sql (PersistFieldSql(..))
import ClassyPrelude.Yesod
import Data.Aeson

data TemporalUnity = Hour | Day | Week | Month deriving (Generic)

instance FromJSON TemporalUnity
instance ToJSON TemporalUnity

data WorkDuration = WorkDurationUndefined
                  | WorkDurationEstimated { workDurationEstimatedUnity :: TemporalUnity
                                          , workDurationEstimatedQuantity :: Int }
                  | WorkDurationPermanent deriving (Generic)

instance FromJSON WorkDuration
instance ToJSON WorkDuration

-- Store the values as as SqlString. The actual column type will be dependent on the database backend. SqlString will be something like varchar/text types.
-- You can also use PersistByteString, which might be faster
instance PersistFieldSql WorkDuration where
  sqlType _ = SqlString


instance PersistField WorkDuration where
  toPersistValue = toPersistValueJSON
  fromPersistValue = fromPersistValueJSON

这是从 Postgres 中选择的样子:

foo=# SELECT * FROM "user";
 id | ident  | password |                                               duration
----+--------+----------+-------------------------------------------------------------------------------------------------------
  5 | foo123 |          | {"tag":"WorkDurationPermanent","contents":[]}
  6 | bar456 |          | {"tag":"WorkDurationEstimated","workDurationEstimatedQuantity":1,"workDurationEstimatedUnity":"Hour"}

我不太熟悉 MySQL 或 Postgres 中的原生 JSON 支持，但如果您想基于底层 JSON 数据进行查询，您可能需要研究一下。