Only render Releases link for project milestones

This commit fixes a bug that was causing a 500 error when a group
milestone was associated to more than 3 releases. This was caused by
attempting to access `milestone.project`, even in the context of a group
milestone.

app/views/shared/milestones/_milestone.html.haml

@@ -25,7 +25,11 @@
               •
           - if total_count > recent_releases.count
             •
-            = link_to n_('%{count} more release', '%{count} more releases', more_count) % { count: more_count }, project_releases_path(milestone.project)
+            - more_text = n_('%{count} more release', '%{count} more releases', more_count) % { count: more_count }
+            - if milestone.project_milestone?
+              = link_to more_text, project_releases_path(milestone.project)
+            - else
+              = more_text
       %div
         = render('shared/milestone_expired', milestone: milestone)
         - if milestone.group_milestone?

ee/changelogs/unreleased/nfriend-fix-301002-group-milestone-bug.yml

+---
+title: Fix 500 error on group milestones page when milestones are associated to more
+  than 3 releases
+merge_request: 55540
+author:
+type: fixed

ee/spec/views/shared/milestones/_milestone.html.haml_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe 'shared/milestones/_milestone.html.haml' do
+  let_it_be(:group) { create(:group) }
+  let_it_be(:project) { create(:project, group: group) }
+  let_it_be(:user) { create(:user).tap { |user| project.add_maintainer(user) } }
+  let_it_be(:releases) { create_list(:release, 4, project: project) }
+  let_it_be(:milestone) { nil }
+
+  let(:more_text) { '1 more release' }
+  let(:link_href) { project_releases_path(project) }
+
+  before do
+    stub_licensed_features(group_milestone_project_releases: true)
+
+    allow(view).to receive(:current_user).and_return(user)
+    allow(view).to receive(:milestone).and_return(milestone)
+    allow(view).to receive(:issues_path).and_return('path/to/issues')
+    allow(view).to receive(:merge_requests_path).and_return('path/to/merge_requests')
+  end
+
+  context 'when a milestone is associated to a lot of releases' do
+    context 'when viewing a project milestone' do
+      let(:milestone) { create(:milestone, project: project, releases: releases) }
+
+      before do
+        assign(:project, project)
+      end
+
+      it 'renders "1 more release" as a link to the project\'s Releases page' do
+        render
+
+        expect(rendered).to have_link(more_text, href: link_href)
+      end
+    end
+
+    context 'when viewing a group milestone' do
+      let(:milestone) { create(:milestone, group: group, releases: releases) }
+
+      before do
+        assign(:group, group)
+      end
+
+      it 'renders "1 more release" as plain text instead of as a link', :aggregate_failures do
+        render
+
+        expect(rendered).not_to have_link(more_text, href: link_href)
+        expect(rendered).to have_content(more_text)
+      end
+    end
+  end
+end