Proper NULL handling in max_, min_ and sum_.

- return expr (Value (Maybe a))
- add joinV to join nested Maybes in an expr

src/Database/Esqueleto.hs

@@ -40,7 +40,7 @@ module Database.Esqueleto
     -- * @esqueleto@'s Language
     Esqueleto( where_, on, groupBy, orderBy, asc, desc, limit, offset, having
              , sub_select, sub_selectDistinct, (^.), (?.)
-             , val, isNothing, just, nothing, countRows, count, not_
+             , val, isNothing, just, nothing, joinV, countRows, count, not_
              , (==.), (>=.), (>.), (<=.), (<.), (!=.), (&&.), (||.)
              , (+.), (-.), (/.), (*.)
              , random_, sum_, round_, ceiling_, floor_, avg_, min_, max_

src/Database/Esqueleto/Internal/Language.hs

@@ -208,6 +208,10 @@ class (Functor query, Applicative query, Monad query) =>
   -- | @NULL@ value.
   nothing :: expr (Value (Maybe typ))
 
+  -- | Join nested 'Maybe's in a 'Value' into one. This is useful when
+  -- calling aggregate functions on nullable fields.
+  joinV :: expr (Value (Maybe (Maybe typ))) -> expr (Value (Maybe typ))
+
   -- | @COUNT(*)@ value.
   countRows :: Num a => expr (Value a)
 
@@ -236,9 +240,9 @@ 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 a)
-  min_     :: (PersistField a) => expr (Value a) -> expr (Value a)
-  max_     :: (PersistField a) => expr (Value a) -> expr (Value a)
+  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)
 
   -- | @LIKE@ operator.

src/Database/Esqueleto/Internal/Sql.hs

@@ -313,6 +313,7 @@ instance Esqueleto SqlQuery SqlExpr SqlBackend where
   isNothing (ERaw p f) = ERaw Parens $ first ((<> " IS NULL") . parensM p) . f
   just (ERaw p f) = ERaw p f
   nothing   = unsafeSqlValue "NULL"
+  joinV (ERaw p f) = ERaw p f
   countRows = unsafeSqlValue "COUNT(*)"
   count (ERaw _ f) = ERaw Never $ \conn -> let (b, vals) = f conn
                                            in ("COUNT" <> parens b, vals)
@@ -1432,7 +1433,7 @@ to16 ((a,b),(c,d),(e,f),(g,h),(i,j),(k,l),(m,n),(o,p)) = (a,b,c,d,e,f,g,h,i,j,k,
 
 -- | Apply extra @SqlExpr Value@ arguments to a 'PersistField' constructor
 (<&>) :: SqlExpr (Insertion (a -> b)) -> SqlExpr (Value a) -> SqlExpr (Insertion b)
-(EInsert _ f) <&> (ERaw _ g) = EInsert Proxy $ \x-> 
+(EInsert _ f) <&> (ERaw _ g) = EInsert Proxy $ \x->
   let (fb, fv) = f x
       (gb, gv) = g x
   in (fb <> ", " <> gb, fv ++ gv)

test/Test.hs

@@ -274,10 +274,10 @@ main = do
           _ <- insert' p4
           ret <- select $
                  from $ \p->
-                 return $ sum_ (p ^. PersonAge)
-          liftIO $ ret `shouldBe` [ Value (36 + 17 + 17 :: Int) ]
+                 return $ joinV $ sum_ (p ^. PersonAge)
+          liftIO $ ret `shouldBe` [ Value $ Just (36 + 17 + 17 :: Int) ]
 
-      it "works with sum_" $
+      it "works with avg_" $
         run $ do
           _ <- insert' p1
           _ <- insert' p2
@@ -296,8 +296,8 @@ main = do
           _ <- insert' p4
           ret <- select $
                  from $ \p->
-                 return $ min_ (p ^. PersonAge)
-          liftIO $ ret `shouldBe` [ Value (17 :: Int) ]
+                 return $ joinV $ min_ (p ^. PersonAge)
+          liftIO $ ret `shouldBe` [ Value $ Just (17 :: Int) ]
 
       it "works with max_" $
         run $ do
@@ -307,8 +307,8 @@ main = do
           _ <- insert' p4
           ret <- select $
                  from $ \p->
-                 return $ max_ (p ^. PersonAge)
-          liftIO $ ret `shouldBe` [ Value (36 :: Int) ]
+                 return $ joinV $ max_ (p ^. PersonAge)
+          liftIO $ ret `shouldBe` [ Value $ Just (36 :: Int) ]
 
       it "works with random_" $
         run $ do