Handle NULL result in avg_ too.

src/Database/Esqueleto.hs

@@ -43,7 +43,8 @@ module Database.Esqueleto
              , val, isNothing, just, nothing, joinV, countRows, count, not_
              , (==.), (>=.), (>.), (<=.), (<.), (!=.), (&&.), (||.)
              , (+.), (-.), (/.), (*.)
-             , random_, sum_, round_, ceiling_, floor_, avg_, min_, max_
+             , random_, round_, ceiling_, floor_
+             , min_, max_,_sum_, avg_,
              , like, (%), concat_, (++.)
              , subList_select, subList_selectDistinct, valList
              , in_, notIn, exists, notExists

src/Database/Esqueleto/Internal/Language.hs

@@ -240,10 +240,11 @@ class (Functor query, Applicative query, Monad query) =>
   round_   :: (PersistField a, PersistField b) => expr (Value a) -> expr (Value b)
   ceiling_ :: (PersistField a, PersistField b) => expr (Value a) -> expr (Value b)
   floor_   :: (PersistField a, PersistField b) => expr (Value a) -> expr (Value b)
+
   sum_     :: (PersistField a) => expr (Value a) -> expr (Value (Maybe a))
   min_     :: (PersistField a) => expr (Value a) -> expr (Value (Maybe a))
   max_     :: (PersistField a) => expr (Value a) -> expr (Value (Maybe a))
-  avg_     :: (PersistField a, PersistField b) => expr (Value a) -> expr (Value b)
+  avg_     :: (PersistField a, PersistField b) => expr (Value a) -> expr (Value (Maybe b))
 
   -- | @LIKE@ operator.
   like :: (PersistField s, IsString s) => expr (Value s) -> expr (Value s) -> expr (Value Bool)

test/Test.hs

@@ -285,8 +285,8 @@ main = do
           _ <- insert' p4
           ret <- select $
                  from $ \p->
-                 return $ avg_ (p ^. PersonAge)
-          liftIO $ ret `shouldBe` [ Value ((36 + 17 + 17) / 3 :: Double) ]
+                 return $ joinV $ avg_ (p ^. PersonAge)
+          liftIO $ ret `shouldBe` [ Value $ Just ((36 + 17 + 17) / 3 :: Double) ]
 
       it "works with min_" $
         run $ do