为持久性实体创建ToJSON实例

Question

我正在尝试使用selectRepYesod 1.2中的新功能,但是我无法让json响应工作.

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)

我从上面的代码得到的错误是

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)

似乎持久性确实为ToJSON (Entity e) 这里提供了一个实例,但我可以用ToJSON (Entity Feed)某种方式吗？

Answer 1

如果您提供实例Feed,则可以使用内置Entity e实例.添加json到您的实体行将自动创建该实例,请参阅:https: //github.com/yesodweb/persistent/wiki/Persistent-entity-syntax#json-instances