save properly

app/views/exercises/_form.html.slim

@@ -29,11 +29,23 @@
       = f.check_box(:allow_file_creation)
       = t('activerecord.attributes.exercise.allow_file_creation')
   h2 = t('activerecord.attributes.exercise.files')
+
+  /----------------------
+  /todo
+  /ul#files.list-unstyled.panel-group
+    = f.fields_for :files do |files_form|
+      = render('file_form', f: files_form)
+
+
   ul#files.list-unstyled.panel-group
-    - @exercise.files.each do |file|
-      = render partial: 'file_form', locals: {file: file}
+    - @exercise.files.each_with_index do |file, index|
+      = f.fields_for(:files) do |files_form|
+        - if index == (files_form.index - 1)
+          = render('file_form', file: file, f: files_form)
+  /----------------------
+
   a#add-file.btn.btn-default.btn-sm.pull-right href='#' = t('.add_file')
-  ul#dummies.hidden
-    = render partial: 'file_form', locals: {file: CodeOcean::File.new}
+  ul#dummies.hidden = f.fields_for(:files, CodeOcean::File.new, child_index: 'index') do |files_form|
+    = render('file_form', file: CodeOcean::File.new,  f: files_form)
   .actions = render('shared/submit_button', f: f, object: @exercise)