Disallow to modify the end date for an ended poll

app/models/poll.rb

@@ -40,6 +40,7 @@ class Poll < ApplicationRecord
   validate :start_date_is_not_past_date, on: :create
   validate :start_date_change, on: :update
   validate :end_date_is_not_past_date, on: :update
+  validate :end_date_change, on: :update
   validate :only_one_active, unless: :public?
 
   accepts_nested_attributes_for :questions, reject_if: :all_blank, allow_destroy: true
@@ -168,6 +169,12 @@ class Poll < ApplicationRecord
     end
   end
 
+  def end_date_change
+    if will_save_change_to_ends_at? && ends_at_in_database < Time.current
+      errors.add(:ends_at, I18n.t("errors.messages.cannot_change_date.poll_ended"))
+    end
+  end
+
   def generate_slug?
     slug.nil?
   end