X-Git-Url: http://gitweb.michael.orlitzky.com/?a=blobdiff_plain;f=src%2FTSN%2FXmlImport.hs;h=022ddad0bb381e2976e049b671141c3bcd7642d0;hb=1f260c118e8da5679820c8cfa489d8fe4a521140;hp=3791ab56ef19add5ec4d0638d94a4e26cec51b51;hpb=e1344533476e1d8111d4228941e0a46ae34e9055;p=dead%2Fhtsn-import.git

diff --git a/src/TSN/XmlImport.hs b/src/TSN/XmlImport.hs
index 3791ab5..022ddad 100644
--- a/src/TSN/XmlImport.hs
+++ b/src/TSN/XmlImport.hs
@@ -1,6 +1,8 @@
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE GADTs #-}
 
+-- | Definition of the XmlImport class.
+--
 module TSN.XmlImport (
   XmlImport(..) )
 where
@@ -14,6 +16,20 @@ 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'
@@ -26,3 +42,18 @@ class (FromXml a, PersistEntity (Db a)) => XmlImport a where
                   => 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