Use a Gitlab::FakeApplicationSettings when migrations are pending

Signed-off-by: Rémy Coutable <remy@rymai.me>

lib/gitlab/current_settings.rb

@@ -47,7 +47,7 @@ module Gitlab
         # defaults for missing columns.
         if ActiveRecord::Migrator.needs_migration?
           db_attributes = current_settings&.attributes || {}
-          ::ApplicationSetting.build_from_defaults(db_attributes)
+          fake_application_settings(db_attributes)
         elsif current_settings.present?
           current_settings
         else

spec/lib/gitlab/current_settings_spec.rb

@@ -116,8 +116,7 @@ describe Gitlab::CurrentSettings do
             let(:current_settings) { described_class.current_application_settings }
 
             it 'returns a non-persisted ApplicationSetting object' do
-              expect(current_settings).to be_a(ApplicationSetting)
-              expect(current_settings).not_to be_persisted
+              expect(current_settings).to be_a(Gitlab::FakeApplicationSettings)
             end
 
             it 'uses the default value from ApplicationSetting.defaults' do
@@ -146,6 +145,16 @@ describe Gitlab::CurrentSettings do
             it 'uses the value from the DB attribute if present and not overridden by an accessor' do
               expect(current_settings.home_page_url).to eq(db_settings.home_page_url)
             end
+
+            context 'when a new column is used before being migrated' do
+              before do
+                allow(ApplicationSetting).to receive(:defaults).and_return({ foo: 'bar' })
+              end
+
+              it 'uses the default value if present' do
+                expect(current_settings.foo).to eq('bar')
+              end
+            end
           end
         end