diff --git a/Database/Persist/TH.hs b/Database/Persist/TH.hs
--- a/Database/Persist/TH.hs
+++ b/Database/Persist/TH.hs
@@ -440,7 +440,7 @@
             (NormalB $ lens' `AppE` getId `AppE` setId)
             []
     if entitySum t
-        then return [idClause]
+        then return $ idClause : map (toSumClause lens' getVal dot keyName valName xName) (entityFields t)
         else return $ idClause : map (toClause lens' getVal dot keyName valName xName) (entityFields t)
   where
     toClause lens' getVal dot keyName valName xName f = Clause
@@ -457,6 +457,27 @@
             $ ConE 'Entity `AppE` VarE keyName `AppE` RecUpdE
                 (VarE valName)
                 [(fieldName, VarE xName)]
+
+    toSumClause lens' getVal dot keyName valName xName f = Clause
+        [ConP (mkName $ unpack $ unHaskellName (entityHaskell t) ++ upperFirst (unHaskellName $ fieldHaskell f)) []]
+        (NormalB $ lens' `AppE` getter `AppE` setter)
+        []
+      where
+        fieldName = mkName $ unpack $ recName (unHaskellName $ entityHaskell t) (unHaskellName $ fieldHaskell f)
+        getter = LamE
+            [ ConP 'Entity [WildP, VarP valName]
+            ] $ CaseE (VarE valName)
+            [ Match (ConP (sumConstrName t f) [VarP xName]) (NormalB $ VarE xName) []
+
+            -- FIXME It would be nice if the types expressed that the Field is
+            -- a sum type and therefore could result in Maybe.
+            , Match WildP (NormalB $ VarE 'error `AppE` LitE (StringL "Tried to use fieldLens on a Sum type")) []
+            ]
+        setter = LamE
+            [ ConP 'Entity [VarP keyName, WildP]
+            , VarP xName
+            ]
+            $ ConE 'Entity `AppE` VarE keyName `AppE` (ConE (sumConstrName t f) `AppE` VarE xName)
 
 mkEntity :: MkPersistSettings -> EntityDef -> Q [Dec]
 mkEntity mps t = do
diff --git a/persistent-template.cabal b/persistent-template.cabal
--- a/persistent-template.cabal
+++ b/persistent-template.cabal
@@ -1,5 +1,5 @@
 name:            persistent-template
-version:         1.1.2.3
+version:         1.1.2.4
 license:         MIT
 license-file:    LICENSE
 author:          Michael Snoyman <michael@snoyman.com>