Change issue show page to group MRs by projects and namespaces

app/controllers/projects/issues_controller.rb

@@ -121,8 +121,7 @@ class Projects::IssuesController < Projects::ApplicationController
   end
 
   def referenced_merge_requests
-    @merge_requests = @issue.referenced_merge_requests(current_user)
-    @closed_by_merge_requests = @issue.closed_by_merge_requests(current_user)
+    @merge_requests, @closed_by_merge_requests = ::Issues::FetchReferencedMergeRequestsService.new(project, current_user).execute(issue)
 
     respond_to do |format|
       format.json do

app/services/issues/fetch_referenced_merge_requests_service.rb

+module Issues
+  class FetchReferencedMergeRequestsService < Issues::BaseService
+    def execute(issue)
+      referenced_merge_requests = issue.referenced_merge_requests(current_user)
+      referenced_merge_requests = Gitlab::IssuableSorter.sort(project, referenced_merge_requests) { |i| i.iid.to_s }
+      closed_by_merge_requests = issue.closed_by_merge_requests(current_user)
+      closed_by_merge_requests = Gitlab::IssuableSorter.sort(project, closed_by_merge_requests) { |i| i.iid.to_s }
+
+      [referenced_merge_requests, closed_by_merge_requests]
+    end
+  end
+end

changelogs/unreleased/26466-natural-sort-mrs.yml

+---
+title: Group MRs on issue page by project and namespace.
+merge_request: 8494
+author: Jeff Stubler

spec/services/issues/fetch_referenced_merge_requests_service_spec.rb

+require 'spec_helper.rb'
+
+describe Issues::FetchReferencedMergeRequestsService do
+  let(:project) { create(:project) }
+  let(:issue) { create(:issue, project: project) }
+  let(:other_project) { create(:project) }
+
+  let(:mr) { create(:merge_request, source_project: project, target_project: project, id: 2)}
+  let(:other_mr) { create(:merge_request, source_project: other_project, target_project: other_project, id: 1)}
+
+  let(:user) { create(:user) }
+  let(:service) { described_class.new(project, user) }
+
+  context 'with mentioned merge requests' do
+    it 'returns a list of sorted merge requests' do
+      allow(issue).to receive(:referenced_merge_requests).with(user).and_return([other_mr, mr])
+
+      mrs, closed_by_mrs = service.execute(issue)
+
+      expect(mrs).to match_array([mr, other_mr])
+      expect(closed_by_mrs).to match_array([])
+    end
+  end
+
+  context 'with closed-by merge requests' do
+    it 'returns a list of sorted merge requests' do
+      allow(issue).to receive(:closed_by_merge_requests).with(user).and_return([other_mr, mr])
+
+      mrs, closed_by_mrs = service.execute(issue)
+
+      expect(mrs).to match_array([])
+      expect(closed_by_mrs).to match_array([mr, other_mr])
+    end
+  end
+end