diff --git a/src/Model/Types/JSON.hs b/src/Model/Types/JSON.hs
index 8d937da6b..655c95294 100644
--- a/src/Model/Types/JSON.hs
+++ b/src/Model/Types/JSON.hs
@@ -22,29 +22,35 @@ import Language.Haskell.TH.Datatype
 derivePersistFieldJSON :: Name -> DecsQ
 derivePersistFieldJSON n = do
   DatatypeInfo{..} <- reifyDatatype n
-  case datatypeVars of
-    [] -> [d|
-      instance PersistField $(conT n) where
-        toPersistValue = PersistDbSpecific . LBS.toStrict . JSON.encode
-        fromPersistValue (PersistDbSpecific bs) = first pack $ JSON.eitherDecodeStrict' bs
-        fromPersistValue (PersistByteString bs) = first pack $ JSON.eitherDecodeStrict' bs
-        fromPersistValue (PersistText       t ) = first pack . JSON.eitherDecodeStrict' $ Text.encodeUtf8 t
-        fromPersistValue _ = Left "JSON values must be converted from PersistDbSpecific, PersistText, or PersistByteString"
-
-      instance PersistFieldSql $(conT n) where
-        sqlType _ = SqlOther "json"
-      |]
-    _  -> do
-      vars <- forM datatypeVars (const $ newName "a")
-      let t = foldl (\t n -> t `appT` conT n) (conT n) vars
-      [d|
-        instance (ToJSON $(t), FromJSON $(t)) => PersistField $(t) where
-          toPersistValue = PersistDbSpecific . LBS.toStrict . JSON.encode
-          fromPersistValue (PersistDbSpecific bs) = first pack $ JSON.eitherDecodeStrict' bs
-          fromPersistValue (PersistByteString bs) = first pack $ JSON.eitherDecodeStrict' bs
-          fromPersistValue (PersistText       t ) = first pack . JSON.eitherDecodeStrict' $ Text.encodeUtf8 t
-          fromPersistValue _ = Left "JSON values must be converted from PersistDbSpecific, PersistText, or PersistByteString"
-
-        instance PersistFieldSql $(t) where
-          sqlType _ = SqlOther "json"
-        |]
+  vars <- forM datatypeVars (const $ newName "a")
+  let t = foldl (\t n -> t `appT` varT n) (conT n) vars
+      iCxt
+        | null vars = cxt []
+        | otherwise = cxt [[t|ToJSON|] `appT` t, [t|FromJSON|] `appT` t]
+      sqlCxt
+        | null vars = cxt []
+        | otherwise = cxt [[t|PersistField|] `appT` t]
+  sequence
+    [ instanceD iCxt ([t|PersistField|] `appT` t)
+      [ funD (mkName "toPersistValue")
+        [ clause [] (normalB [e|PersistDbSpecific . LBS.toStrict . JSON.encode|]) []
+        ]
+      , funD (mkName "fromPersistValue")
+        [ do
+            bs <- newName "bs"
+            clause [[p|PersistDbSpecific $(varP bs)|]] (normalB [e|first pack $ JSON.eitherDecodeStrict' $(varE bs)|]) []
+        , do
+            bs <- newName "bs"
+            clause [[p|PersistByteString $(varP bs)|]] (normalB [e|first pack $ JSON.eitherDecodeStrict' $(varE bs)|]) []
+        , do
+            t <- newName "t"
+            clause [[p|PersistText $(varP t)|]] (normalB [e|first pack . JSON.eitherDecodeStrict' $ Text.encodeUtf8 $(varE t)|]) []
+        , clause [wildP] (normalB [e|Left "JSON values must be converted from PersistDbSpecific, PersistText, or PersistByteString"|]) []
+        ]
+      ]
+    , instanceD sqlCxt ([t|PersistFieldSql|] `appT` t)
+      [ funD (mkName "sqlType")
+        [ clause [wildP] (normalB [e|SqlOther "json"|]) []
+        ]
+      ]
+    ]