diff --git a/src/Handler/Material.hs b/src/Handler/Material.hs
index ca7a05b62..a0e2097b7 100644
--- a/src/Handler/Material.hs
+++ b/src/Handler/Material.hs
@@ -13,6 +13,9 @@ import Utils.Lens
 import Utils.Form
 import Handler.Utils
 
+import Control.Monad.Writer (MonadWriter(..), execWriterT)
+
+
 
 data MaterialForm = MaterialForm
       { mfName         :: MaterialName
@@ -141,10 +144,18 @@ handleMaterialEdit tid ssh csh cid template dbMaterial = do
 
     insertMaterialFile' :: MaterialId -> Source Handler (Either FileId File) -> DB ()
     insertMaterialFile' mid fs = do
-      oldFileIds <- E.select . E.from $ \(file `E.InnerJoin` materialFile) -> do
-        error "TODO"
-
-      error "TODO"
+        oldFileIdVals <- E.select . E.from $ \(file `E.InnerJoin` materialFile) -> do
+          E.on     $  materialFile E.^. MaterialFileFile E.==. file E.^. FileId
+          E.where_ $  materialFile E.^. MaterialFileMaterial E.==. E.val mid
+          return $ file E.^. FileId
+        let oldFileIds = setFromList $ map E.unValue oldFileIdVals
+        keep <- execWriterT . runConduit $ transPipe (lift . lift) fs =$= C.mapM_ finsert
+        mapM_ deleteCascade $ (oldFileIds \\ keep :: Set FileId)
+      where
+        finsert (Left fileId) = tell $ singleton fileId
+        finsert (Right file) = lift $ do
+          fid <- insert file
+          void . insert $ MaterialFile mid fid -- cannot fail due to uniqueness, since we generated a fresh FileId in the previous step
 
 
 getMDelR, postMDelR :: TermId -> SchoolId -> CourseShorthand ->  MaterialName -> Handler Html