diff --git a/mu-persistent.cabal b/mu-persistent.cabal
--- a/mu-persistent.cabal
+++ b/mu-persistent.cabal
@@ -1,5 +1,5 @@
 name:               mu-persistent
-version:            0.2.0.0
+version:            0.3.0.0
 synopsis:           Utilities for interoperation between Mu and Persistent
 description:
   Please see the <https://github.com/higherkindness/mu-haskell/persistent#readme readme file>.
@@ -29,7 +29,9 @@
   build-depends:
       base          >=4.7   && <5
     , monad-logger
-    , mu-schema     >=0.2.0
+    , mu-schema     >=0.3.0
     , persistent
     , resourcet
     , transformers
+
+  ghc-options:      -Wall
diff --git a/src/Mu/Adapter/Persistent.hs b/src/Mu/Adapter/Persistent.hs
--- a/src/Mu/Adapter/Persistent.hs
+++ b/src/Mu/Adapter/Persistent.hs
@@ -1,13 +1,13 @@
-{-# language DataKinds             #-}
-{-# language FlexibleContexts      #-}
-{-# language FlexibleInstances     #-}
-{-# language GADTs                 #-}
-{-# language KindSignatures        #-}
-{-# language MultiParamTypeClasses #-}
-{-# language ScopedTypeVariables   #-}
-{-# language TypeApplications      #-}
-{-# language TypeOperators         #-}
-{-# language UndecidableInstances  #-}
+{-# language DataKinds              #-}
+{-# language FlexibleContexts       #-}
+{-# language FlexibleInstances      #-}
+{-# language FunctionalDependencies #-}
+{-# language GADTs                  #-}
+{-# language KindSignatures         #-}
+{-# language ScopedTypeVariables    #-}
+{-# language TypeApplications       #-}
+{-# language TypeOperators          #-}
+{-# language UndecidableInstances   #-}
 {-|
 Description : Utilities for interoperation between Mu and Persistent
 
@@ -28,14 +28,13 @@
 import           Control.Monad.Logger
 import           Control.Monad.Trans.Reader
 import           Control.Monad.Trans.Resource.Internal
-import           Data.Functor.Identity
 import           Data.Int
 import           Database.Persist.Sql
 import           GHC.Generics
 import           GHC.TypeLits
+
 import           Mu.Schema
 import           Mu.Schema.Class
-import           Mu.Schema.Interpretation
 
 -- | Wrapper for 'Entity' to be used with @DerivingVia@.
 --   This wrappers indicates that the identifier is to be found
@@ -53,36 +52,40 @@
 newtype WithEntityPlainId (ty :: Symbol) (fmap :: Mappings Symbol Symbol) a
   = WithEntityPlainId { unWithEntityPlainId :: a }
 
-instance ( Generic t, Applicative w
+instance ( Generic t
          , (sch :/: sty) ~ 'DRecord name (idArg ': args)
          , idArg ~ 'Mu.Schema.FieldDef idArgName ('TPrimitive Int64)
          , Rep t ~ D1 dInfo (C1 cInfo f)
-         , GToSchemaRecord Identity sch fmap args f
+         , GToSchemaRecord sch fmap args f
          , ToBackendKey (PersistEntityBackend t) t
          , PersistEntityBackend t ~ SqlBackend )
-         => ToSchema w sch sty (WithEntityPlainId sty fmap (Entity t)) where
+         => ToSchema sch sty (WithEntityPlainId sty fmap (Entity t)) where
   toSchema (WithEntityPlainId (Entity key x))
-    = TRecord $ Field (pure $ FPrimitive (unSqlBackendKey $ toBackendKey key))
-      :* transFieldsNoMaps up (toSchemaRecord (Proxy @fmap) (unM1 $ unM1 $ from x))
-    where up :: Identity a -> w a
-          up (Identity i) = pure i
+    = TRecord $ Field (FPrimitive (unSqlBackendKey $ toBackendKey key))
+      :* toSchemaRecord (Proxy @fmap) (unM1 $ unM1 $ from x)
 
-instance ( Generic t, Applicative w
+instance ( Generic t
          , (sch :/: sty) ~ 'DRecord name (nestedIdArg ': args)
-         , nestedIdArg ~ 'Mu.Schema.FieldDef fname ('TSchematic idTy)
+         , nestedIdArg ~ 'Mu.Schema.FieldDef fname k
+         , ToSchemaKey sch idTy k
          , (sch :/: idTy) ~ 'DRecord idName '[idArg]
          , idArg ~ 'Mu.Schema.FieldDef idArgName ('TPrimitive Int64)
          , Rep t ~ D1 dInfo (C1 cInfo f)
-         , GToSchemaRecord Identity sch fmap args f
+         , GToSchemaRecord sch fmap args f
          , ToBackendKey (PersistEntityBackend t) t
          , PersistEntityBackend t ~ SqlBackend )
-         => ToSchema w sch sty (WithEntityNestedId sty fmap (Entity t)) where
+         => ToSchema sch sty (WithEntityNestedId sty fmap (Entity t)) where
   toSchema (WithEntityNestedId (Entity key x))
-    = TRecord $ Field (pure $ FSchematic $ TRecord (Field (pure $ FPrimitive key') :* Nil))
-      :* transFieldsNoMaps up (toSchemaRecord (Proxy @fmap) (unM1 $ unM1 $ from x))
+    = TRecord $ Field (toSchemaKey $ FSchematic $ TRecord (Field (FPrimitive key') :* Nil))
+      :* toSchemaRecord (Proxy @fmap) (unM1 $ unM1 $ from x)
     where key' = unSqlBackendKey $ toBackendKey key
-          up :: Identity a -> w a
-          up (Identity i) = pure i
+
+class ToSchemaKey (sch :: Schema') (idTy :: Symbol) t | sch t -> idTy where
+  toSchemaKey :: FieldValue sch ('TSchematic idTy) -> FieldValue sch t
+instance ToSchemaKey sch idTy ('TSchematic idTy) where
+  toSchemaKey = id
+instance ToSchemaKey sch idTy t => ToSchemaKey sch idTy ('TOption t) where
+  toSchemaKey = FOption . Just . toSchemaKey
 
 -- | Simple utility to execute a database operation
 --   in any monad which supports 'IO' operations.