update default attachment when delete attachment

app/controllers/v1/learning_objects/attachment_controller.rb

@@ -36,7 +36,15 @@ class V1::LearningObjects::AttachmentController < ApplicationController
     return render status: :not_found if @learning_object.nil? || @attachment.nil?
 
     DeleteBitstreamWorker.perform_async(@attachment.id)
-    @attachment.destroy
+    if @learning_object.attachment == @attachment
+      @learning_object.attachment = nil
+      @learning_object.save
+      @attachment.destroy
+      @learning_object.reload
+      @learning_object.default_attachment
+    else
+      @attachment.destroy
+    end
 
     render status: :ok
   end

db/migrate/20171205120824_update_learning_object_attachments_foreign_key.rb

+class UpdateLearningObjectAttachmentsForeignKey < ActiveRecord::Migration[5.0]
+  def change
+    remove_foreign_key :learning_object_attachments, :learning_object_attachments
+    add_foreign_key :learning_object_attachments, :learning_object_attachments, column: :attachment_id, on_delete: :cascade
+  end
+end