add getExportedKey

Not optimised because that would need transition code to be written for
existing export datbases.

Database/Export.hs

@@ -19,6 +19,7 @@ module Database.Export (
 	addExportedLocation,
 	removeExportedLocation,
 	getExportedLocation,
+	getExportedKey,
 	isExportDirectoryEmpty,
 	getExportTreeCurrent,
 	recordExportTreeCurrent,
@@ -154,6 +155,20 @@ getExportedLocation (ExportHandle h _) k = H.queryDbQueue h $ do
   where
 	ik = toIKey k
 
+{- Get the key that was exported to a location.
+ -
+ - Note that the database does not currently have an index to make this
+ - fast.
+ -
+ - Note that this does not see recently queued changes.
+ -}
+getExportedKey :: ExportHandle -> ExportLocation -> IO [Key]
+getExportedKey ExportHandle h _) el = H.queryDbQueue h $ do
+	l <- selectList [ExportedFile ==. ef] []
+	return $ map (fromSKey . exportedKey . entityVal) l
+  where
+	ef = toSFilePath (fromExportLocation el)
+
 {- Note that this does not see recently queued changes. -}
 isExportDirectoryEmpty :: ExportHandle -> ExportDirectory -> IO Bool
 isExportDirectoryEmpty (ExportHandle h _) d = H.queryDbQueue h $ do