Restore fallback to deployment_platform_cluster

In 12.2 we will remove this fallback.

app/models/deployment_metrics.rb

@@ -44,7 +44,18 @@ class DeploymentMetrics
     end
   end
 
+  # TODO remove fallback case to deployment_platform_cluster.
+  # Otherwise we will continue to pay the performance penalty described in
+  # https://gitlab.com/gitlab-org/gitlab-ce/issues/63475
+  #
+  # Removal issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/64105
   def cluster_prometheus
-    cluster.application_prometheus if cluster&.application_prometheus_available?
+    cluster_with_fallback = cluster || deployment_platform_cluster
+
+    cluster_with_fallback.application_prometheus if cluster_with_fallback&.application_prometheus_available?
+  end
+
+  def deployment_platform_cluster
+    deployment.environment.deployment_platform&.cluster
   end
 end

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -887,9 +887,10 @@ describe Projects::MergeRequestsController do
         environment2 = create(:environment, project: forked)
         create(:deployment, :succeed, environment: environment2, sha: sha, ref: 'master', deployable: build)
 
-        # TODO address the last 5 queries
-        # See https://gitlab.com/gitlab-org/gitlab-ce/issues/63952
-        leeway = 5
+        # TODO address the last 11 queries
+        # See https://gitlab.com/gitlab-org/gitlab-ce/issues/63952 (5 queries)
+        # And https://gitlab.com/gitlab-org/gitlab-ce/issues/64105 (6 queries)
+        leeway = 11
         expect { get_ci_environments_status }.not_to exceed_all_query_limit(control_count + leeway)
       end
 

spec/models/deployment_metrics_spec.rb

@@ -49,6 +49,18 @@ describe DeploymentMetrics do
 
         it { is_expected.to be_truthy }
       end
+
+      context 'fallback deployment platform' do
+        let(:cluster) { create(:cluster, :provided_by_user, environment_scope: '*', projects: [deployment.project]) }
+        let!(:prometheus) { create(:clusters_applications_prometheus, :installed, cluster: cluster) }
+
+        before do
+          expect(deployment.project).to receive(:deployment_platform).and_return(cluster.platform)
+          expect(cluster.application_prometheus).to receive(:can_query?).and_return(true)
+        end
+
+        it { is_expected.to be_truthy }
+      end
     end
   end