Test Prometheus proxy client generation

app/models/clusters/applications/prometheus.rb

@@ -40,15 +40,20 @@ module Clusters
       end
 
       def proxy_client
-        return unless cluster.kubeclient
+        return unless kube_client
 
-        kube_client = cluster.kubeclient
         proxy_url = kube_client.proxy_url('service', service_name, service_port, Gitlab::Kubernetes::Helm::NAMESPACE)
 
         # ensures headers containing auth data are appended to original k8s client options
         options = kube_client.rest_client.options.merge(headers: kube_client.headers)
         RestClient::Resource.new(proxy_url, options)
       end
+
+      private
+
+      def kube_client
+        cluster&.kubeclient
+      end
     end
   end
 end

spec/models/clusters/applications/prometheus_spec.rb

@@ -31,4 +31,55 @@ describe Clusters::Applications::Prometheus do
       expect(subject).to eq("#{Rails.root}/vendor/prometheus/values.yaml")
     end
   end
+
+  describe '#proxy_client' do
+    context 'cluster is nil' do
+      it 'returns nil' do
+        expect(subject.cluster).to be_nil
+        expect(subject.proxy_client).to be_nil
+      end
+    end
+
+    context "cluster doesn't have kubeclient" do
+      let(:cluster) { create(:cluster) }
+      subject { create(:clusters_applications_prometheus, cluster: cluster) }
+
+      it 'returns nil' do
+        expect(subject.proxy_client).to be_nil
+      end
+    end
+
+    context 'cluster has kubeclient' do
+      let(:kubernetes_url) { 'http://example.com' }
+      let(:k8s_discover_response) { {
+        resources: [
+          {
+            name: 'service',
+            kind: 'Service'
+          }]
+      } }
+
+      let(:kube_client) { Kubeclient::Client.new(kubernetes_url) }
+
+      let(:cluster) { create(:cluster) }
+      subject { create(:clusters_applications_prometheus, cluster: cluster) }
+
+      before do
+        allow(kube_client.rest_client).to receive(:get).and_return(k8s_discover_response.to_json)
+        allow(subject.cluster).to receive(:kubeclient).and_return(kube_client)
+      end
+
+      it 'creates proxy prometheus rest client' do
+        expect(subject.proxy_client).to be_instance_of(RestClient::Resource)
+      end
+
+      it 'creates proper url' do
+        expect(subject.proxy_client.url).to eq('http://example.com/api/v1/proxy/namespaces/gitlab-managed-apps/service/prometheus-prometheus-server:80')
+      end
+
+      it 'copies options and headers from kube client to proxy client' do
+        expect(subject.proxy_client.options).to eq(kube_client.rest_client.options.merge(headers: kube_client.headers))
+      end
+    end
+  end
 end