Util.hs

-- |
-- Module      : Database.PostgreSQL.Simple.Util
-- Copyright   : (c) 2014 Andreas Meingast <ameingast@gmail.com>
--
-- License     : BSD-style
-- Maintainer  : andre@andrevdm.com
-- Stability   : experimental
-- Portability : GHC
--
-- A collection of utilites for database migrations.

{-# LANGUAGE OverloadedStrings #-}

module Database.PostgreSQL.Simple.Util
    ( existsTable
    , withTransactionRolledBack
    ) where

import           Control.Exception ( finally )
import           Database.PostgreSQL.Simple ( Connection
                                            , Only (..)
                                            , begin
                                            , query
                                            , rollback
                                            )
import           GHC.Int (Int64)

-- | Checks if the table with the given name exists in the database.
existsTable :: Connection -> String -> IO Bool
existsTable con table =
  checkRowCount <$> (query con q (Only table) :: IO [[Int64]])
  where
    q = "select count(relname) from pg_class where relname = ?"

    checkRowCount :: [[Int64]] -> Bool
    checkRowCount ((1:_):_) = True
    checkRowCount _ = False

-- | Executes the given IO monad inside a transaction and performs a roll-back
-- afterwards (even if exceptions occur).
withTransactionRolledBack :: Connection -> IO a -> IO a
withTransactionRolledBack con f =
  begin con >> finally f (rollback con)