Add persistence exception handling section

Cookbook.md

@@ -38,6 +38,7 @@ For non-Yesod code, see also [Snippets](https://github.com/yesodweb/yesod-cookbo
 * [Example MySQL Connection code](https://github.com/yesodweb/yesod-cookbook/blob/master/cookbook/Example-MySQL-Connection-code.md)
 * [Activate foreign key checking in Sqlite](https://github.com/yesodweb/yesod-cookbook/blob/master/cookbook/Activate-foreign-key-checking-in-Sqlite.md)
 * [Adding Seed data to Scaffolded Site](https://github.com/yesodweb/yesod-cookbook/blob/master/cookbook/Adding-Seed-Data-to-Scaffolded-Site.md)
+* [Handling exception in persistence](https://github.com/yesodweb/yesod-cookbook/blob/master/cookbook/Handling-Persistence-Exception.md)
 
 ## State
 

cookbook/Handling-Persistence-Exception.md

@@ -0,0 +1,82 @@
+# Handling exception in persistence
+
+Say, you have a code like this which throws exception:
+
+``` haskell
+share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persistLowerCase|
+User
+    age Int
+    UniqueAge age
+    deriving Show
+|]
+
+getUser :: MonadIO m => ReaderT SqlBackend m [Entity User]
+getUser = selectList [] []
+
+insertJane :: MonadIO m => ReaderT SqlBackend m ()
+insertJane = insert_ $ User 40
+
+sqliteTest :: IO ()
+sqliteTest = runSqlite ":memory:" $ do
+               runMigration migrateAll
+
+               insertJane
+               insertJane
+
+               users <- getUser
+               liftIO $ print (users :: [Entity User])
+```
+
+The above code will throw exception when executed because the uniqueness constraint is being violated:
+
+```
+$ ./sqlite-code
+
+Migrating: CREATE TABLE "user"("id" INTEGER PRIMARY KEY,"age" INTEGER NOT NULL,CONSTRAINT "unique_age" UNIQUE ("age"))
+persistent-try-bugs: SQLite3 returned ErrorConstraint while attempting to perform step.
+```
+
+You can use the [exceptions](https://hackage.haskell.org/package/exceptions) package to handle exceptions. Have the appropriate imports:
+
+```
+import Control.Exception (SomeException)
+import Control.Monad.Catch
+```
+
+And the rest of the code is like this:
+
+``` haskell
+share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persistLowerCase|
+User
+    age Int
+    UniqueAge age
+    deriving Show
+|]
+
+getUser :: MonadIO m => ReaderT SqlBackend m [Entity User]
+getUser = selectList [] []
+
+insertJane :: (MonadCatch m, MonadIO m) => ReaderT SqlBackend m ()
+insertJane = (insert_ (User 40)) `catch` (\(SomeException e) -> return ())
+
+sqliteTest :: IO ()
+sqliteTest = runSqlite ":memory:" $ do
+               runMigration migrateAll
+
+               insertJane
+               insertJane
+
+               users <- getUser
+               liftIO $ print (users :: [Entity User])
+```
+
+And this time, it will execute without crashing:
+
+``` 
+$ ./sqlite-code
+
+Migrating: CREATE TABLE "user"("id" INTEGER PRIMARY KEY,"age" INTEGER NOT NULL,CONSTRAINT "unique_age" UNIQUE ("age"))
+[Entity {entityKey = UserKey {unUserKey = SqlBackendKey {unSqlBackendKey = 1}}, entityVal = User {userAge = 40}}]
+```
+
+