Fix viewing blobs for broken MRs

Render code-nav definition path even if head sha
does not exist

app/helpers/diff_helper.rb

@@ -254,7 +254,7 @@ module DiffHelper
   end
 
   def code_navigation_path(diffs)
-    Gitlab::CodeNavigationPath.new(merge_request.project, diffs.diff_refs&.head_sha)
+    Gitlab::CodeNavigationPath.new(merge_request.project, merge_request.diff_head_sha)
   end
 
   def conflicts

app/serializers/diffs_entity.rb

@@ -79,7 +79,9 @@ class DiffsEntity < Grape::Entity
   end
 
   expose :definition_path_prefix do |diffs|
-    project_blob_path(merge_request.project, diffs.diff_refs&.head_sha)
+    next unless merge_request.diff_head_sha
+
+    project_blob_path(merge_request.project, merge_request.diff_head_sha)
   end
 
   def merge_request

changelogs/unreleased/id-fix-no-route-error.yml

+---
+title: Fix viewing blobs for broken MRs
+merge_request: 52483
+author:
+type: fixed

spec/serializers/diffs_entity_spec.rb

@@ -3,10 +3,11 @@
 require 'spec_helper'
 
 RSpec.describe DiffsEntity do
-  let(:user) { create(:user) }
-  let(:project) { create(:project, :repository) }
+  let_it_be(:user) { create(:user) }
+  let_it_be(:project) { create(:project, :repository) }
+  let_it_be(:merge_request) { create(:merge_request_with_diffs, target_project: project, source_project: project) }
+
   let(:request) { EntityRequest.new(project: project, current_user: user) }
-  let(:merge_request) { create(:merge_request_with_diffs, target_project: project, source_project: project) }
   let(:merge_request_diffs) { merge_request.merge_request_diffs }
   let(:options) do
     { request: request, merge_request: merge_request, merge_request_diffs: merge_request_diffs }
@@ -30,6 +31,14 @@ RSpec.describe DiffsEntity do
       )
     end
 
+    context 'broken merge request' do
+      let(:merge_request) { create(:merge_request, :invalid, target_project: project, source_project: project) }
+
+      it 'renders without errors' do
+        expect { subject }.not_to raise_error
+      end
+    end
+
     context "when a commit_id is passed" do
       let(:commits) { merge_request.commits }
       let(:entity) do