Add uniqueness validation for votes

app/models/vote.rb

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

spec/models/vote_spec.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Vote do
+  let(:user) { create(:user) }
+  let(:event) { create(:event) }
+  let(:vote) { create(:vote, event: event, user: user) }
+
+  describe 'validation' do
+    it 'has a valid factory' do
+      expect(build(:vote)).to be_valid
+    end
+
+    it 'validates uniqueness of event in scope of user' do
+      expect(build(:vote, event: vote.event, user: vote.user)).not_to be_valid
+    end
+  end
+end