Validate Stage xor Group present in match

app/models/match.rb

@@ -1,9 +1,17 @@
 # frozen_string_literal: true
 
 class Match < ApplicationRecord
-  belongs_to :stage
-  belongs_to :group
+  belongs_to :stage, optional: true
+  belongs_to :group, optional: true
   has_many :scores, dependent: :destroy
 
   validates :scores, length: { maximum: 2 }
+
+  validate :stage_xor_group
+
+  private
+
+  def stage_xor_group
+    errors.add(:stage_xor_group, 'Stage and Group missing or both present') unless stage.present? ^ group.present?
+  end
 end

spec/factories/matches.rb

@@ -1,8 +1,11 @@
 # frozen_string_literal: true
 
 FactoryBot.define do
-  factory :match do
+  factory :stage_match, aliases: [:match], class: Match do
     stage
+  end
+
+  factory :group_match, class: Match do
     group
   end
 end

spec/models/match_spec.rb

@@ -9,6 +9,23 @@ RSpec.describe Match, type: :model do
     it { should belong_to :group }
   end
 
+  context '#new' do
+    it 'needs only a group' do
+      match = Match.new group: build(:group)
+      expect(match).to be_valid
+    end
+
+    it 'needs only a stage' do
+      match = Match.new stage: build(:stage)
+      expect(match).to be_valid
+    end
+
+    it 'can\'t have a group and a stage' do
+      match = Match.new group: build(:group), stage: build(:stage)
+      expect(match).to be_invalid
+    end
+  end
+
   context 'scores' do
     before do
       @match = create(:match)