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

spec/models/vote_spec.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Vote do
+  let!(:vote) { create(:vote) }
+
+  describe 'validation' do
+    it 'has a valid factory' do
+      expect(build(:vote)).to be_valid
+    end
+
+    it { is_expected.to validate_uniqueness_of(:user_id).scoped_to(:event_id) }
+
+    # This is testing the relationship instead of using the shoulda-matchers
+    context 'vote with user already exists' do
+      it 'fails when adding vote twice for user and event' do
+        expect { create(:vote, user: vote.user, event: vote.event) }
+          .to raise_error(ActiveRecord::RecordInvalid, 'Validation failed: User has already been taken')
+      end
+    end
+  end
+end