Customize JSON instances

User.hs

-- | Goal is not to expose sensitive user data such as the field email.
--
-- When creating the user, the request body only has: username, email, bio (optional), image (optional)
-- When returning the user, I only want to show: username, bio, image.
--
-- Error:
-- /Users/rap/code/thesis/ninja/src/Api.hs:52:11: error:
--    • No instance for (aeson-1.0.2.1:Data.Aeson.Types.ToJSON.ToJSON
--                         (persistent-2.6.1:Database.Persist.Class.PersistEntity.Entity
--                            Models.User.User))
--        arising from a use of ‘serve’
--    • In the expression: serve appApi (appToServer cfg)
--      In an equation for ‘app’: app cfg = serve appApi (appToServer cfg)
--
-- I'm a bit puzzled by this.
--
-- I came acrros this wiki entry, but I can't understand how to use mpsEntityJSON.
-- https://github.com/yesodweb/persistent/wiki/Persistent-entity-syntax#json-instances

share [mkPersist sqlSettings, mkMigrate "migrateAll"] [persistLowerCase|
User sql=users
    username  Text
    email     Text
    bio       Text Maybe default=NULL
    image     Text Maybe default=NULL
    createdAt UTCTime default=now()
    updatedAt UTCTime Maybe default=NULL

    UniqueUser username email
    deriving Show Generic
|]

instance FromJSON User where
  parseJSON = withObject "User" $ \ v ->
    User <$> v .:  "username"
         <*> v .:  "email"
         <*> v .:? "bio"
         <*> v .:? "image"
         <*> v .:  "createdAt"
         <*> v .:? "updatedAt"

instance ToJSON User where
    toJSON User {userUsername, userBio, userImage} =
        object [ "username" .= userUsername
               , "bio"      .= userBio
               , "image"    .= userImage
               ]