Association.hs

import           Database.Persist.Sql
import qualified Database.Persist.TH  as DPTH
--Consider the followering database structure

DPTH.share [DPTH.mkPersist DPTH.sqlSettings, DPTH.mkMigrate "migrateAll"] [DPTH.persistLowerCase|
  User
    name T.Text
  UserPost
    user_id UserId
    post_id PostId
  Post
    author_id UserId
    content Text
  Follower
    user_id UserId
    follower_id FollowerId
|]


-- We almost never want to work with just a user here. We usually want to work with a type more like(forgive the double definition of user)
data User = User { posts :: [Entity Post], name :: T.Text, followers :: [Entity Follower] }

-- We have to manually load these associations to create this type so wouldn't it be nice have this taken care of for us?

-- We could create a type class
class (PersistEntity a, PersistEntity b) => HasMany a b where
  data HasMany a b = HasMany a b
  -- define Join table and Foreign key
  
-- So we could write
instance HasMany User Post where
  -- definition
instance HasMany User Follower where
  -- definition
allUsers :: [User]

-- But this is expensive. What if we have 5 has many associations? We dont want to always load every association
-- What we really want is a datatype like
type MaybeLoaded = Maybe
data User = User { posts :: MaybeLoaded [Entity Post], name :: T.Text, followers :: MaybeLoaded [Entity Follower] }

--and a function that we can provide a list of associations to load

allUsers :: PersistEntity b => [HasMany User b] -> [User]