{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE GADTs #-}

-- | Definition of the XmlImport class.
--
module TSN.XmlImport (
  XmlImport(..) )
where

import Database.Groundhog (
  AutoKey,
  insert,
  insertByAll )
import Database.Groundhog.Core ( PersistBackend, PersistEntity )

import Xml ( FromXml(..) )


-- | In Groundhog, there is a typeclass of things you can insert into
--   the database. What we usually have, though, is an XML
--   representation of something that has a Groundhog analogue that we
--   could insert into the database. It would be real nice if we could
--   just insert the XML thing and not have to convert back and
--   forth. That's what the 'XmlImport' class lets you do.
--
--   Moreover, there is a contraint on the class that the type must
--   also be a member of the 'FromXml' class. This allows us to define
--   default implementations of \"insert me\" generically. Given any
--   XML thing that can be converted to a database thing, we just do
--   the conversion and then insert normally (however Groundhog would
--   do it).
--
class (FromXml a, PersistEntity (Db a)) => XmlImport a where
  -- | This is similar to the signature for Groundhog's 'insert'
  --   function, except the 'AutoKey' we return is for our 'Db'
  --   counterpart.
  insert_xml :: (PersistBackend m) => a ->  m (AutoKey (Db a))
  insert_xml x = insert (from_xml x)

  -- | Same rationale as 'insert_xml', except it uses 'insertByAll'.
  insertByAll_xml :: (PersistBackend m)
                  => a
                  -> m ( Either (AutoKey (Db a)) (AutoKey (Db a)) )
  insertByAll_xml x = insertByAll (from_xml x)


  -- | Try to insert the given object and get its primary key
  --   back. Or, if there's a unique constraint violation, get the
  --   primary key of the unique thing already present.
  --
  --   Note: we can switch to using fmap here as soon as Functor is a
  --   superclass of Monad (PersistBackend is a Monad).
  --
  insert_xml_or_select :: (PersistBackend m)
                       => a
                       -> m (AutoKey (Db a))
  insert_xml_or_select x = do
    tmp <- insertByAll_xml x
    return $ (either id id) tmp