Modify Union sql construction

Changelog: fixed

app/models/deployment.rb

@@ -8,7 +8,6 @@ class Deployment < ApplicationRecord
   include Importable
   include Gitlab::Utils::StrongMemoize
   include FastDestroyAll
-  include FromUnion
 
   StatusUpdateError = Class.new(StandardError)
   StatusSyncError = Class.new(StandardError)

app/models/preloaders/environments/deployment_preloader.rb

@@ -21,11 +21,13 @@ module Preloaders
       def load_deployment_association(association_name, association_attributes)
         return unless environments.present?
 
-        union_arg = environments.inject([]) do |result, environment|
-          result << environment.association(association_name).scope
-        end
-
-        union_sql = Deployment.from_union(union_arg).to_sql
+        # Not using Gitlab::SQL::Union as `order_by` in the SQL constructed is ignored.
+        # See:
+        #   1) https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/sql/union.rb#L7
+        #   2) https://gitlab.com/gitlab-org/gitlab/-/issues/353966#note_860928647
+        union_sql = environments.map do |environment|
+          "(#{environment.association(association_name).scope.to_sql})"
+        end.join(' UNION ')
 
         deployments = Deployment
                         .from("(#{union_sql}) #{::Deployment.table_name}")

spec/controllers/projects/environments_controller_spec.rb

@@ -6,7 +6,7 @@ RSpec.describe Projects::EnvironmentsController do
   include MetricsDashboardHelpers
   include KubernetesHelpers
 
-  let_it_be(:project) { create(:project) }
+  let_it_be(:project) { create(:project, :repository) }
   let_it_be(:maintainer) { create(:user, name: 'main-dos').tap { |u| project.add_maintainer(u) } }
   let_it_be(:reporter) { create(:user, name: 'repo-dos').tap { |u| project.add_reporter(u) } }
 
@@ -55,11 +55,11 @@ RSpec.describe Projects::EnvironmentsController do
       let(:environments) { json_response['environments'] }
 
       context 'with default parameters' do
-        before do
-          get :index, params: environment_params(format: :json)
-        end
+        subject { get :index, params: environment_params(format: :json) }
 
         it 'responds with a flat payload describing available environments' do
+          subject
+
           expect(environments.count).to eq 3
           expect(environments.first).to include('name' => 'production', 'name_without_type' => 'production')
           expect(environments.second).to include('name' => 'staging/review-1', 'name_without_type' => 'review-1')
@@ -69,9 +69,28 @@ RSpec.describe Projects::EnvironmentsController do
         end
 
         it 'sets the polling interval header' do
+          subject
+
           expect(response).to have_gitlab_http_status(:ok)
           expect(response.headers['Poll-Interval']).to eq("3000")
         end
+
+        context 'validates latest deployment' do
+          let_it_be(:test_environment) do
+            create(:environment, project: project, name: 'staging/review-4', state: :available)
+          end
+
+          before do
+            create_list(:deployment, 2, :success, environment: test_environment, project: project)
+          end
+
+          it 'responds with the latest deployment for the environment' do
+            subject
+
+            environment = environments.find { |env| env['id'] == test_environment.id }
+            expect(environment['last_deployment']['id']).to eq(test_environment.deployments.last.id)
+          end
+        end
       end
 
       context 'when a folder-based nested structure is requested' do

spec/serializers/environment_serializer_spec.rb

@@ -204,6 +204,25 @@ RSpec.describe EnvironmentSerializer do
 
       json
     end
+
+    # Validates possible bug that can arise when order_by is not honoured in the preloader.
+    # See: https://gitlab.com/gitlab-org/gitlab/-/issues/353966#note_861381504
+    it 'fetches the last and upcoming deployment correctly' do
+      last_deployment = nil
+      upcoming_deployment = nil
+      create(:environment, project: project).tap do |environment|
+        create(:deployment, :success, environment: environment, project: project)
+        last_deployment = create(:deployment, :success, environment: environment, project: project)
+
+        create(:deployment, :running, environment: environment, project: project)
+        upcoming_deployment = create(:deployment, :running, environment: environment, project: project)
+      end
+
+      response_json = json
+
+      expect(response_json.last[:last_deployment][:id]).to eq(last_deployment.id)
+      expect(response_json.last[:upcoming_deployment][:id]).to eq(upcoming_deployment.id)
+    end
   end
 
   def create_environment_with_associations(project)