Hacking Persistent

August 2, 2015

I’m a big fan of persistent as it offers compile time integrity checks for your database. Whenever I need to model a new schema, I usually use this tool to come up with the design. As I model and typecheck the code, I usually use the printMigration function to see the translated SQL queries. The code looks something like this:

{-# LANGUAGE EmptyDataDecls             #-}
{-# LANGUAGE FlexibleContexts           #-}
{-# LANGUAGE GADTs                      #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiParamTypeClasses      #-}
{-# LANGUAGE OverloadedStrings          #-}
{-# LANGUAGE QuasiQuotes                #-}
{-# LANGUAGE TemplateHaskell            #-}
{-# LANGUAGE TypeFamilies               #-}
import Control.Monad.IO.Class  (liftIO)
import Control.Monad.Logger    (runStderrLoggingT)
import Database.Persist
import Database.Persist.Postgresql
import Database.Persist.TH

share [mkPersist sqlSettings, mkMigrate "migrateAll"]
    name String
    age Int Maybe
    address Int
    deriving Show
    title String
    authorId PersonId
    deriving Show

connStr = "host=localhost dbname=test user=test password=test port=5432"

main :: IO ()
main = runStderrLoggingT $ withPostgresqlPool connStr 10 $
       \pool -> liftIO $ do
                  flip runSqlPersistMPool pool $ do
                           printMigration migrateAll

But there is a slight problem with this code. For the program to run successfully, It needs an actual database to be present in the system. With that being in mind, I filed an issue hoping it would be fixed. After around 10 months and with a little push from Michael Snoyman, I finally started working on the patch. As I studied the code, I sent a series of cleanup patches. Finally I figured out the problem with the actual issue and wrote a patch for PostgreSQL driver for it. Right now it is merged and lives in the master. So for the above code to run properly without the actual database being present, make use of the mockMigration function:

main :: IO ()
main = mockMigration migrateAll

And that will print the migration for your designed database. And today I have sent a pull request for the MySQL backend. Once it gets merged, you can also use this functionality for designing schemas using MySQL too. And yes, this is the first time I’m doing the development of Haskell packages using Nix.