From d6703f9646b046848c5e36fa8ce8cea766597f6f Mon Sep 17 00:00:00 2001
From: Paul Brinkmeier <hallo@pbrinkmeier.de>
Date: Wed, 3 Jul 2024 17:00:58 +0200
Subject: [PATCH] Add tuple decoding into multiple records

---
 lib/Database/PostgreSQL/Opium/FromRow.hs | 22 ++++++++++++++++++++++
 test/Database/PostgreSQL/OpiumSpec.hs    |  7 +++++++
 2 files changed, 29 insertions(+)

diff --git a/lib/Database/PostgreSQL/Opium/FromRow.hs b/lib/Database/PostgreSQL/Opium/FromRow.hs
index 60bcc06..02a1589 100644
--- a/lib/Database/PostgreSQL/Opium/FromRow.hs
+++ b/lib/Database/PostgreSQL/Opium/FromRow.hs
@@ -53,12 +53,34 @@ class FromRow a where
   fromRow result columnTable row =
     runExceptT $ to <$> fromRow' @0 FRProxy (FromRowCtx result columnTable) row
 
+instance
+  ( Generic a
+  , GetColumnTable' (Rep a)
+  , FromRow' 0 (Rep a)
+  , Generic b
+  , GetColumnTable' (Rep b)
+  , FromRow' (NumberOfMembers (Rep a)) (Rep b)
+  ) => FromRow (a, b) where
+  getColumnTable Proxy result = runExceptT $ do
+    ctA <- newColumnTable <$> getColumnTable' @(Rep a) Proxy result
+    ctB <- newColumnTable <$> getColumnTable' @(Rep b) Proxy result
+    pure $ ctA `concatColumnTables` ctB
+
+  fromRow result ct row = runExceptT $ do
+    x <- to <$> fromRow' @0 FRProxy (FromRowCtx result ct) row
+    y <- to <$> fromRow' @(NumberOfMembers (Rep a)) FRProxy (FromRowCtx result ct) row
+    pure (x, y)
+
 newtype ColumnTable = ColumnTable (Vector (Column, Oid))
   deriving (Eq, Show)
 
 newColumnTable :: [(Column, Oid)] -> ColumnTable
 newColumnTable = ColumnTable . Vector.fromList
 
+concatColumnTables :: ColumnTable -> ColumnTable -> ColumnTable
+concatColumnTables (ColumnTable a) (ColumnTable b) =
+  ColumnTable $ a <> b
+
 indexColumnTable :: ColumnTable -> Int -> (Column, Oid)
 indexColumnTable (ColumnTable v) i = v `Vector.unsafeIndex` i
 
diff --git a/test/Database/PostgreSQL/OpiumSpec.hs b/test/Database/PostgreSQL/OpiumSpec.hs
index f62a62f..7983ee4 100644
--- a/test/Database/PostgreSQL/OpiumSpec.hs
+++ b/test/Database/PostgreSQL/OpiumSpec.hs
@@ -122,6 +122,13 @@ spec = do
       row <- Opium.fromRow result columnTable 0
       row `shouldBe` Right (ManyFields "abc" 42 1.0 "test" True)
 
+    it "Decodes multiple records into a tuple" $ \conn -> do
+      Just result <- LibPQ.execParams conn "SELECT 'albus' AS name, 123 AS age, 42 AS only" [] LibPQ.Binary
+      Right columnTable <- Opium.getColumnTable @(Person, Only Int) Proxy result
+
+      row <- Opium.fromRow @(Person, Only Int) result columnTable 0
+      row `shouldBe` Right (Person "albus" 123, Only 42)
+
   describe "fetch" $ do
     it "Passes numbered parameters and retrieves a list of rows" $ \conn -> do
       rows <- Opium.fetch "SELECT ($1 + $2) AS only" (17 :: Int, 25 :: Int) conn