Get id from Entity record - haskell

Question

I have an Entity record, specifically Entity User and I need to extract the Id which the User has in the database as an Int.
From reading the docs it seems entityKey would be useful here but I'm not quite sure how I would go about getting an Int out.

Answer 1

You have to use a combination of fromSqlKey and entityKey. A sample program demonstrating it:
#!/usr/bin/env stack
{- stack
--resolver lts-9.0
--install-ghc
runghc
--package persistent
--package persistent-sqlite
--package persistent-template
--package mtl
-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE EmptyDataDecls #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE QuasiQuotes #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies #-}
import Control.Monad.IO.Class (liftIO, MonadIO)
import Control.Monad.Reader
import Database.Persist
import Database.Persist.Sqlite
import Database.Persist.TH
import Data.Int
share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persistLowerCase|
Person
name String
age Int Maybe
deriving Show
|]
insertPerson :: MonadIO m => ReaderT SqlBackend m (Key Person)
insertPerson = insert $ Person "Michael" $ Just 26
main :: IO ()
main = runSqlite ":memory:" $ do
runMigration migrateAll
michaelId <- insertPerson
(michael :: Entity Person) <- getJustEntity michaelId
liftIO $ print $ (fromSqlKey . entityKey $ michael :: Int64)
And it's output:
~/g/scripts $ stack persist.hs
Migrating: CREATE TABLE "person"("id" INTEGER PRIMARY KEY,"name" VARCHAR NOT NULL,"age" INTEGER NULL)
1