Introduce #find_sidebar and use a constant to find the page

app/controllers/projects/wikis_controller.rb

@@ -107,7 +107,7 @@ class Projects::WikisController < Projects::ApplicationController
     # Call #wiki to make sure the Wiki Repo is initialized
     @project_wiki.wiki
 
-    @sidebar_page = @project_wiki.find_page('_sidebar', params[:version_id])
+    @sidebar_page = @project_wiki.find_sidebar(params[:version_id])
 
     unless @sidebar_page # Fallback to default sidebar
       @sidebar_wiki_entries = WikiPage.group_by_directory(@project_wiki.pages(limit: 15))

app/models/project_wiki.rb

@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class ProjectWiki
   include Gitlab::ShellAdapter
   include Storage::LegacyProjectWiki
@@ -9,6 +11,7 @@ class ProjectWiki
   }.freeze unless defined?(MARKUPS)
 
   CouldNotCreateWikiError = Class.new(StandardError)
+  SIDEBAR = '_sidebar'
 
   # Returns a string describing what went wrong after
   # an operation fails.
@@ -95,6 +98,10 @@ class ProjectWiki
     end
   end
 
+  def find_sidebar(version = nil)
+    find_page(SIDEBAR, version)
+  end
+
   def find_file(name, version = nil)
     wiki.file(name, version)
   end

spec/models/project_wiki_spec.rb

@@ -170,6 +170,22 @@ describe ProjectWiki do
     end
   end
 
+  describe '#find_sidebar' do
+    before do
+      create_page(described_class::SIDEBAR, 'This is an awesome Sidebar')
+    end
+
+    after do
+      subject.pages.each { |page| destroy_page(page.page) }
+    end
+
+    it 'finds the page defined as _sidebar' do
+      page = subject.find_page('_sidebar')
+
+      expect(page.content).to eq('This is an awesome Sidebar')
+    end
+  end
+
   describe '#find_file' do
     shared_examples 'finding a wiki file' do
       before do