Create ToJSON instance for persistent Entity

Question

I'm trying to use the new selectRep function from Yesod 1.2, but I'm having trouble getting json responses to work.

instance ToJSON (Entity Feed) where
    toJSON (Entity uid (Feed url lastUpdated)) = object 
        [ "id" .= uid
        , "url" .= url
        , "lastUpdated" .= lastUpdated
        ]

getFeedByIdR :: FeedId -> Handler TypedContent
getFeedByIdR feedId = do
    feed <- runDB $ get404 feedId
    selectRep $ do
        provideRep $ return $ toJSON (Entity feedId feed)

The error I get from the above code is

Handler/Feed.hs:23:31:
    Overlapping instances for ToJSON (Entity Feed)
      arising from a use of `toJSON'
    Matching instances:
      instance ToJSON e => ToJSON (Entity e)
        -- Defined in `persistent-1.2.0.1:Database.Persist.Class.PersistEntity'
      instance ToJSON (Entity Feed) -- Defined at Handler/Feed.hs:5:10
    In the second argument of `($)', namely
      `toJSON (Entity feedId feed)'
    In the second argument of `($)', namely
      `return $ toJSON (Entity feedId feed)'
    In a stmt of a 'do' block:
      provideRep $ return $ toJSON (Entity feedId feed)

It seems that persistent does indeed provide an instance for ToJSON (Entity e) here, but can I use my ToJSON (Entity Feed) somehow?

Answer 1

If you provide an instance for Feed, then you can use the built-in Entity e instance. Adding json to your entity line will create that instance automatically, see: https://github.com/yesodweb/persistent/wiki/Persistent-entity-syntax#json-instances