Add a method in Project to return a cached value of total count of projects

This is in preparation to address the DB load caused by the counting in
gitlab-com/infrastructure#303.

app/models/project.rb

@@ -378,6 +378,12 @@ class Project < ActiveRecord::Base
 
       joins(join_body).reorder('join_note_counts.amount DESC')
     end
+
+    def cached_count
+      Rails.cache.fetch('total_project_count', expires_in: 5.minutes) do
+        Project.count
+      end
+    end
   end
 
   def repository_storage_path

spec/models/project_spec.rb

@@ -714,6 +714,20 @@ describe Project, models: true do
     it { expect(project.builds_enabled?).to be_truthy }
   end
 
+  describe '.cached_count', caching: true do
+    let(:group)     { create(:group, :public) }
+    let!(:project1) { create(:empty_project, :public, group: group) }
+    let!(:project2) { create(:empty_project, :public, group: group) }
+
+    it 'returns total project count' do
+      expect(Project).to receive(:count).once.and_call_original
+
+      3.times do
+        expect(Project.cached_count).to eq(2)
+      end
+    end
+  end
+
   describe '.trending' do
     let(:group)    { create(:group, :public) }
     let(:project1) { create(:empty_project, :public, group: group) }

spec/spec_helper.rb

@@ -42,6 +42,13 @@ RSpec.configure do |config|
   config.before(:suite) do
     TestEnv.init
   end
+
+  config.around(:each, :caching) do |example|
+    caching_store = Rails.cache
+    Rails.cache = ActiveSupport::Cache::MemoryStore.new if example.metadata[:caching]
+    example.run
+    Rails.cache = caching_store
+  end
 end
 
 FactoryGirl::SyntaxRunner.class_eval do