From de2d9f8a0bd0aeb0b19eafb16c64be8fd2071e64 Mon Sep 17 00:00:00 2001
From: Fintan Halpenny <rossmanf@tcd.ie>
Date: Sun, 23 Jul 2017 23:49:34 +0100
Subject: [PATCH] Got a working example of cascading delete but it requires a
 select followed by a delete

---
 examples/Main.hs | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/examples/Main.hs b/examples/Main.hs
index 5c85cf3..e9d63f6 100644
--- a/examples/Main.hs
+++ b/examples/Main.hs
@@ -5,6 +5,7 @@
 {-# LANGUAGE MultiParamTypeClasses      #-}
 {-# LANGUAGE OverloadedStrings          #-}
 {-# LANGUAGE QuasiQuotes                #-}
+{-# LANGUAGE ScopedTypeVariables        #-}
 {-# LANGUAGE StandaloneDeriving         #-}
 {-# LANGUAGE TemplateHaskell            #-}
 {-# LANGUAGE TypeFamilies               #-}
@@ -17,6 +18,7 @@ module Main
 -------------------------------------------------------------------------------
 import           Blog
 import           Control.Monad               (void)
+import           Control.Monad               (forM_)
 import           Control.Monad.IO.Class      (MonadIO, liftIO)
 import           Control.Monad.Logger        (MonadLogger)
 import           Control.Monad.Reader        (MonadReader (..), runReaderT)
@@ -31,7 +33,9 @@ import           Database.Persist.TH         (mkDeleteCascade, mkMigrate,
 -------------------------------------------------------------------------------
 
 
-share [mkPersist sqlSettings, mkDeleteCascade sqlSettings, mkMigrate "migrateAll"] [persistLowerCase|
+share [ mkPersist sqlSettings
+      , mkDeleteCascade sqlSettings
+      , mkMigrate "migrateAll"] [persistLowerCase|
   Person
     name String
     age Int Maybe
@@ -136,12 +140,18 @@ updateJoao =
 
 -------------------------------------------------------------------------------
 deleteYoungsters :: (MonadIO m, MonadLogger m)
-                 => SqlWriteT m ()
+                 => SqlPersistT m ()
 deleteYoungsters = do
   -- | Delete any persons under the age of 14
-  delete $
+
+  -- | In this case where `ON DELETE CASCADE` is not generated by migration
+  -- | we select all the entities we want to delete and then for each one
+  -- | one we extract the key and use Persistent's `deleteCascade`
+  youngsters <- select $
     from $ \p -> do
     where_ (p ^. PersonAge <. just (val 14))
+    pure p
+  forM_ youngsters (deleteCascade . entityKey)
 
 
 -------------------------------------------------------------------------------
@@ -198,7 +208,6 @@ cleanDb = do
   where
     dropTable tableName = rawExecute ("DROP TABLE " <> tableName) []
 
-
 -------------------------------------------------------------------------------
 main :: IO ()
 main = do
@@ -229,4 +238,4 @@ main = do
   where
     say :: (MonadIO m, Show a) => a -> m ()
     say = liftIO . print
-    connection = "host=localhost port=5433 user=postgres dbname=esqueleto_blog_example password=Leon93"
+    connection = "host=localhost port=5433 user=postgres dbname=esqueleto_blog_example password=***"