Look for milestone by iid in milestone controller

app/controllers/projects/milestones_controller.rb

@@ -81,7 +81,7 @@ class Projects::MilestonesController < Projects::ApplicationController
   protected
 
   def milestone
-    @milestone ||= @project.milestones.find(params[:id])
+    @milestone ||= @project.milestones.find_by_iid!(params[:id])
   end
 
   def authorize_admin_milestone!

spec/models/milestone_spec.rb

@@ -25,6 +25,7 @@ describe Milestone do
   end
 
   describe "Validation" do
+    before { subject.stub(set_iid: false) }
     it { should validate_presence_of(:title) }
     it { should validate_presence_of(:project) }
   end