relational-record on persisten backends.

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BSD3 licensed by Takahiro HIMURA
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persistent-relational-record build a bridge between Haskell Relational Record and Persistent. It uses the persistent entities definition instead of obtaining schema from DB at compilation time.

Getting Started

If you already define an entities in persistent's manner, then you are almost ready to use this module. The entities definition in the style of persistent-relational-record are shown below:


~~~~ {.haskell} {-# LANGUAGE GADTs #-} {-# LANGUAGE GeneralizedNewtypeDeriving #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE TemplateHaskell #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE QuasiQuotes #-} {-# LANGUAGE FlexibleInstances #-}

import Data.Text (Text) import Database.Persist.Relational (mkHrrInstances) import Database.Persist.TH

share [mkPersist sqlSettings, mkMigrate "migrateAll", mkSave "db", mkHrrInstances] [persistLowerCase| Image title Text deriving Eq Show Tag name Text deriving Eq Show ImageTag imageId ImageId tagId TagId |]


The main difference from the persistent version is that mkSave "db" and mkHrrInstances are added to the 1st argument of the share function. mkSave "db" saves the definition of tables to "db" variable for later use. mkHrrInstances generates various instances from the entities definition to cooperate with HRR.

Next, you should define HRR record types and their instances, this package provides "defineTableFromPersistent" function to generate those types and auxiliary functnions. To avoid the conflict of record field names, we recommend making one module per table.

Here is the content of "Image.hs":

~~~~ {.haskell} {-# LANGUAGE TemplateHaskell #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# OPTIONS_GHC -fno-warn-orphans #-}

module Image where

import Data.Text (Text) import Database.Persist.Relational import Model hiding (Image) -- Both of HRR and persistent generates Image type, so you should hide Image type generated by persistent. import qualified Model

defineTableFromPersistent ''Model.Image db


You should create "Tag.hs" and "ImageTag.hs" in the same manner.

Now, you can build queries in manner of HRR:

~~~~ {.haskell} module Query where

import Data.Text (Text) import Database.Relational.Query

import Model import qualified Image import qualified ImageTag import qualified Tag

imageIdFromTagNameList :: [Text] -- ^ list of tag name -> Relation () ImageId imageIdFromTagNameList tagNames = aggregateRelation $ do imgtag <- query $ ImageTag.imageTag tag <- query $ Tag.tag on $ tag !' .=. imgtag ! ImageTag.tagId' wheres $ tag !' in' values tagNames g <- groupBy $ imgtag ! ImageTag.imageId' let c = count $ imgtag ! ImageTag.imageId' having $ c .=. value (length $ tagNames) return g

selectImageByTagNameList :: [Text] -- ^ list of tag name -> Relation () Image.Image selectImageByTagNameList tagNames = relation $ do img <- query Image.image imgids <- query $ imageIdFromTagNameList tagNames on $ img !' .=. imgids return img


Finally, we can execute a query by runQuery:

~~~~ {.haskell} {-# LANGUAGE OverloadedStrings #-}

import Control.Monad.Base import Control.Monad.Logger import Control.Monad.Trans.Resource import Data.Conduit import qualified Data.Conduit.List as CL import Database.Persist.MySQL import Database.Persist.Relational import Database.Relational.Query

import Model import Query

sample1 :: SqlPersistT (LoggingT IO) [ImageId] sample1 = runResourceT $ runQuery (relationalQuery $ imageIdFromTagNameList ["tokyo", "haskell"]) () $$ CL.consume

sample2 :: SqlPersistT (LoggingT IO) [Entity Image] sample2 = runResourceT $ runQuery (relationalQuery $ selectImageByTagNameList ["tokyo", "haskell"]) () $$ CL.consume

main :: IO () main = runStderrLoggingT $ withMySQLPool defaultConnectInfo 10 $ runSqlPool $ do mapM (liftBase . print) =<< sample1 mapM (liftBase . print) =<< sample2


runQuery run the HRR Query and gives the result as conduit Source. In addition, it converts the result type to persistent's entity if the result type of Query is HRR record type.

For example, the expression selectImageByTagNameList [...] has type Relation () Image.Image, but runQuery (relationalQuery $ selectImageByTagNameList ["tokyo", "haskell"]) () has type Source m (Entity Image).

For a full runnable example, see examples directory.

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