problems with using persistant in a yesod sub site

gistfile1.hs

{-# LANGUAGE MultiParamTypeClasses, TypeFamilies, GeneralizedNewtypeDeriving #-}
{-# LANGUAGE QuasiQuotes, TemplateHaskell, OverloadedStrings #-}
{-# LANGUAGE DeriveDataTypeable, NoMonomorphismRestriction, FlexibleInstances #-}
{-# LANGUAGE FlexibleContexts #-}

import Data.Text (Text)
import Language.Haskell.TH.Syntax hiding (lift)
import Yesod


share2 mkPersist (mkMigrate "migrateTest") [persist|
    Test
        foo Text
|]

data TestSite = TestSite

mkYesodSub "TestSite" [ ClassP ''YesodPersist [VarT $ mkName "master"] ] 
  [parseRoutes|
    /insert InsertR POST
  |]                             


postInsertR :: (YesodPersist master) => GHandler sub master ()
postInsertR = do
  runDB $ insert $ Test "bar"
  return ()
  
{-
test.hs:26:11:
    Could not deduce (PersistBackend
                        (YesodDB master (GGHandler sub master IO)))
      arising from a use of `insert'
    from the context (YesodPersist master)
      bound by the type signature for
                 postInsertR :: YesodPersist master => GHandler sub master ()
      at /home/alios/src/test.hs:(25,1)-(31,2)
    Possible fix:
      add (PersistBackend
             (YesodDB master (GGHandler sub master IO))) to the context of
        the type signature for
          postInsertR :: YesodPersist master => GHandler sub master ()
      or add an instance declaration for
         (PersistBackend (YesodDB master (GGHandler sub master IO)))
    In the expression: insert
    In the second argument of `($)', namely `insert $ Test "bar"'
    In a stmt of a 'do' expression: runDB $ insert $ Test "bar"
Failed, modules loaded: none.
-}

Michael Snoyman's suggestion on the haskell-webdevel mailinglist

{-# LANGUAGE MultiParamTypeClasses, TypeFamilies, GeneralizedNewtypeDeriving #-}
{-# LANGUAGE QuasiQuotes, TemplateHaskell, OverloadedStrings #-}
{-# LANGUAGE DeriveDataTypeable, NoMonomorphismRestriction,
FlexibleInstances #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE UndecidableInstances #-}

import Data.Text (Text)
import Language.Haskell.TH.Syntax hiding (lift)
import Yesod


share2 mkPersist (mkMigrate "migrateTest") [persist|
   Test
       foo Text
|]

data TestSite = TestSite

mkYesodSub "TestSite"
   [ ClassP ''YesodPersist [VarT $ mkName "master"]
   , ClassP ''PersistBackend
       [ ConT ''YesodDB
           `AppT` VarT (mkName "master")
           `AppT` (ConT ''GGHandler `AppT` ConT ''TestSite `AppT`
VarT (mkName "master") `AppT` ConT ''IO)
       ]
   ]
 [parseRoutes|
   /insert InsertR POST
 |]


postInsertR :: (YesodPersist master, PersistBackend (YesodDB master
(GGHandler sub master IO))) => GHandler sub master ()
postInsertR = do
 runDB $ insert $ Test "bar"
 return ()