[web-devel] [ Newbie ] [ MongoDB ] Using MongoDB Persistent backend

[Michael Snoyman]

On Wed, Jun 29, 2011 at 11:17 AM, Kamil Ciemniewski
<ciemniewski.kamil at gmail.com> wrote:
> Hi all,
>
> I've got some "newbie" problems using MongoDB as backend
> in Persistent.
>
> Basically I've got my app type defined as:
> data MyApp = MyApp
>     { getStatic :: Static -- ^ Settings for static file serving.
>     , connPool :: ConnPool Host
>     }
>
> And I made it an instance of YesodPersist by:
> instance YesodPersist MyApp where
>   type YesodDB MyApp = MongoDBReader Host
>   runDB db = liftIOHandler
>            $ fmap connPool getYesod >>= (\p -> return (p, "localhost")) >>=
> runMongoDBConn db
>
> And I made it an instance of YesodAuth as well by:
> instance YesodAuth MyApp where
>   type AuthId MyApp = UserId
>
>   loginDest _ = RootR
>   logoutDest _ = RootR
>
>   getAuthId creds = runDB $ do
>       x <- getBy $ UniqueUser $ credsIdent creds
>       case x of
>       Just (uid, _) -> return $ Just uid
>       Nothing -> do
>           fmap Just $ insert $ User (credsIdent creds) Nothing
>
>   authPlugins = [ authEmail ]
>
> I've defined User model as yesod scaffold tool defines it.
>
> Now, when I try to compile it i get:
>
> No instances for (Control.Monad.Context.Context
>                         Database.MongoDB.Connection.MasterOrSlaveOk
>                         (GGHandler s MyApp IO),
>                       Control.Monad.Context.Context
>                         Database.MongoDB.Query.Database (GGHandler s MyApp
> IO),
>                       Control.Monad.Context.Context
>                         Database.MongoDB.Internal.Protocol.Pipe (GGHandler s
> MyApp IO),
>                       Control.Monad.Context.Context
>                         Database.MongoDB.Query.WriteMode (GGHandler s MyApp
> IO),
>                       Control.Monad.Throw.Throw
>                         Database.MongoDB.Query.Failure (GGHandler s MyApp
> IO))
>       arising from a use of `insert'
>     Possible fix:
>       add instance declarations for
>       (Control.Monad.Context.Context
>          Database.MongoDB.Connection.MasterOrSlaveOk
>          (GGHandler s MyApp IO),
>        Control.Monad.Context.Context
>          Database.MongoDB.Query.Database (GGHandler s MyApp IO),
>        Control.Monad.Context.Context
>          Database.MongoDB.Internal.Protocol.Pipe (GGHandler s MyApp IO),
>        Control.Monad.Context.Context
>          Database.MongoDB.Query.WriteMode (GGHandler s MyApp IO),
>        Control.Monad.Throw.Throw
>          Database.MongoDB.Query.Failure (GGHandler s MyApp IO))
>     In the expression: insert
>     In the second argument of `($)', namely
>       `insert $ User (credsIdent creds) Nothing'
>     In the expression:
>         fmap Just $ insert $ User (credsIdent creds) Nothing
>
> The problem is probably very trivial.. But I've got no ideas how to fix it.
>
> Best regards
> Kamil Ciemniewski
>
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>
>

I don't have direct experience with the MongoDB backend, but I can
give this a shot if I saw some code. I think one possibility would be
to wrap everything in a liftIO, but then you won't have access to the
Handler monad features. This would prevent you from doing things like
setting messages and sending redirects from the database access
itself, but should otherwise be fine.

Michael