Add uniqueness tests for Event scope on Vote model with User association

app/models/vote.rb

@@ -4,6 +4,8 @@ class Vote < ApplicationRecord
   belongs_to :user
   belongs_to :event
 
+  validates :user_id, uniqueness: { scope: :event_id }
+
   has_paper_trail ignore: [:updated_at], meta: { conference_id: :conference_id }
 
   delegate :name, to: :user