Test that alert's environment is verified

Check that creating an alert for an project non-specific environment
is not possible and return 204.

ee/app/controllers/projects/prometheus/alerts_controller.rb

@@ -30,7 +30,7 @@ module Projects
       def create
         @alert = project.prometheus_alerts.create(alerts_params)
 
-        if @alert
+        if @alert.persisted?
           schedule_prometheus_update!
 
           render json: serialize_as_json(@alert)

ee/changelogs/unreleased/pl-test-invalid-creation.yml

+---
+title: Return proper status code when creation of an alert fails
+merge_request: 7360
+author: Peter Leitzen
+type: fixed

ee/spec/controllers/projects/prometheus/alerts_controller_spec.rb

@@ -146,6 +146,21 @@ describe Projects::Prometheus::AlertsController do
       expect(response).to have_gitlab_http_status(200)
       expect(JSON.parse(response.body)).to include(alert_params)
     end
+
+    context 'with a project non-specific environment' do
+      let(:environment) { create(:environment) }
+
+      it 'returns 204 status' do
+        post :create, project_params(
+          operator: ">",
+          threshold: "1",
+          environment_id: environment.id,
+          prometheus_metric_id: metric.id
+        )
+
+        expect(response).to have_gitlab_http_status(:no_content)
+      end
+    end
   end
 
   describe 'POST #update' do