Developing Web Apps with Haskell and Yesod, 2nd Edition

Chapter 19. SQL Joins

Persistent touts itself as a database-agnostic interface. How, then, are you supposed to do things that are inherently backend-specific? This most often comes up in Yesod when you want to join two tables together. There are some pure-Haskell solutions that are completely backend-agonistic, but there are also more efficient methods at our disposal. In this chapter, we’ll introduce a common problem you might want to solve, and then build up more sophisticated solutions.

Multiauthor Blog

Blogs are a well-understood problem domain, so let’s use that for our problem setup. Consider a blog engine that allows for multiple authors in the database, but supports blog posts that have a single author. In Persistent, we may model this as:

Author
    name Text
Blog
    author AuthorId
    title Text
    content Html

Let’s set up our initial Yesod application to show a blog post index indicating the blog title and the author:

{-# LANGUAGE EmptyDataDecls             #-}
{-# LANGUAGE FlexibleContexts           #-}
{-# LANGUAGE GADTs                      #-}
{-# LANGUAGE GeneralizedNewtypeDeriving #-}
{-# LANGUAGE MultiParamTypeClasses      #-}
{-# LANGUAGE OverloadedStrings          #-}
{-# LANGUAGE QuasiQuotes                #-}
{-# LANGUAGE TemplateHaskell            #-}
{-# LANGUAGE TypeFamilies               #-}
{-# LANGUAGE ViewPatterns               #-}
import           Control.Monad.Logger
import           Data.Text               (Text)
import           Database.Persist.Sqlite
import           Yesod

share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persistLowerCase|
Author
    name Text
Blog
    author AuthorId
    title Text
    content Html
|]

data