{-# LANGUAGE DeriveDataTypeable #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE TypeFamilies #-}
module Model.Cache ( getCached ) where

import Data.Typeable ( Typeable )
import Database.Persist ( Entity (..), Key (..), PersistStore, PersistRecordBackend, keyToValues )
import Yesod ( MonadHandler, HandlerSite, YesodPersist, YesodPersistBackend, cachedBy, get404, liftHandlerT, runDB )

import qualified Data.ByteString.Char8 as C

newtype CachedEntity t = CachedEntity { unCachedEntity :: Entity t } deriving Typeable

getCached :: ( MonadHandler m
             , YesodPersist (HandlerSite m)
             , PersistStore (YesodPersistBackend (HandlerSite m))
             , PersistRecordBackend entity (YesodPersistBackend (HandlerSite m))
             , Typeable entity
             ) => Key entity -> m (Entity entity)
getCached entId = liftHandlerT . cached . runDB . withId . get404 $ entId
    where key = C.pack . show . keyToValues $ entId
          withId = fmap $ Entity entId
          cached = fmap unCachedEntity . cachedBy key . fmap CachedEntity