Fix environment deployment platform filter method

app/models/environment.rb

@@ -224,7 +224,7 @@ class Environment < ActiveRecord::Base
   end
 
   def deployment_platform
-    project.deployment_platform(environment: self)
+    project.deployment_platform(environment: self.name)
   end
 
   private

lib/gitlab/prometheus/queries/query_additional_metrics.rb

@@ -79,7 +79,7 @@ module Gitlab
         def common_query_context(environment, timeframe_start:, timeframe_end:)
           base_query_context(timeframe_start, timeframe_end).merge({
             ci_environment_slug: environment.slug,
-            kube_namespace: environment.project.deployment_platform&.actual_namespace || '',
+            kube_namespace: environment.deployment_platform&.actual_namespace || '',
             environment_filter: %{container_name!="POD",environment="#{environment.slug}"}
           })
         end

spec/models/environment_spec.rb

@@ -368,6 +368,32 @@ describe Environment do
     end
   end
 
+  describe '#deployment_platform' do
+    context 'when there is a deployment platform for environment' do
+      let!(:cluster) do
+        create(:cluster, :provided_by_gcp,
+               environment_scope: '*', projects: [project])
+      end
+
+      it 'finds a deployment platform' do
+        expect(environment.deployment_platform).to eq cluster.platform
+      end
+    end
+
+    context 'when there is no deployment platform for environment' do
+      it 'returns nil' do
+        expect(environment.deployment_platform).to be_nil
+      end
+    end
+
+    it 'checks deployment platforms associated with a project' do
+      expect(project).to receive(:deployment_platform)
+        .with(environment: environment.name)
+
+      environment.deployment_platform
+    end
+  end
+
   describe '#terminals' do
     subject { environment.terminals }