Add Compare support to UrlBuilder

lib/gitlab/url_builder.rb

@@ -24,6 +24,8 @@ module Gitlab
           instance.project_job_url(object.project, object, **options)
         when Commit
           commit_url(object, **options)
+        when Compare
+          compare_url(object, **options)
         when Group
           instance.group_canonical_url(object, **options)
         when Issue
@@ -68,6 +70,12 @@ module Gitlab
         instance.commit_url(commit, **options)
       end
 
+      def compare_url(compare, **options)
+        return '' unless compare.project
+
+        instance.project_compare_url(compare.project, **options.merge(compare.to_param))
+      end
+
       def note_url(note, **options)
         if note.for_commit?
           return '' unless note.project

spec/factories/compares.rb

+# frozen_string_literal: true
+
+FactoryBot.define do
+  factory :compare do
+    skip_create # No persistence
+
+    start_project { association(:project, :repository) }
+    target_project { start_project }
+
+    start_ref { 'master' }
+    target_ref { 'feature' }
+
+    base_sha { nil }
+    straight { false }
+
+    initialize_with do
+      CompareService
+        .new(start_project, start_ref)
+        .execute(target_project, target_ref, base_sha: base_sha, straight: straight)
+    end
+  end
+end

spec/lib/gitlab/url_builder_spec.rb

@@ -69,6 +69,27 @@ RSpec.describe Gitlab::UrlBuilder do
       end
     end
 
+    context 'when passing a compare' do
+      # NOTE: The Compare requires an actual repository, which isn't available
+      # with the `build_stubbed` strategy used by the table tests above
+      let_it_be(:compare) { create(:compare) }
+      let_it_be(:project) { compare.project }
+
+      it 'returns the full URL' do
+        expect(subject.build(compare)).to eq("#{Gitlab.config.gitlab.url}/#{project.full_path}/-/compare/#{compare.base_commit_sha}...#{compare.head_commit_sha}")
+      end
+
+      it 'returns only the path if only_path is given' do
+        expect(subject.build(compare, only_path: true)).to eq("/#{project.full_path}/-/compare/#{compare.base_commit_sha}...#{compare.head_commit_sha}")
+      end
+
+      it 'returns an empty string for missing project' do
+        expect(compare).to receive(:project).and_return(nil)
+
+        expect(subject.build(compare)).to eq('')
+      end
+    end
+
     context 'when passing a commit without a project' do
       let(:commit) { build_stubbed(:commit) }