Refactor TrendingProjectsFinder to support cachingtrending-caching

== Public Projects

This finder class now _only_ returns public projects. Previously this
finder would also return private and internal projects. Including these
projects makes caching data much harder and less efficient. Meanwhile
including this data isn't very useful as very few users would be
interested in seeing projects they have access to as trending. That is,
the feature is more useful when you want to see what _other_ popular
projects there are.

== Caching

The data returned by TrendingProjectsFinder is now cached for a day
based on the number of months the data should be restricted to. The
cache is not flushed explicitly, instead it's rebuilt whenever it
expires.

== Timings

To measure the impact I changed the finder code to use the last 24
months instead of the last month. I then executed and measured 10
requests to the explore page. On the current "master" branch (commit
88fa5916ffa0aea491cd339272ed7437c3f52dc7) this would take an average of
2.43 seconds. Using the changes of this commit this was reduced to
around 1.7 seconds.

Fixes gitlab-org/gitlab-ce#22164

4 files changed, 44 insertions, 25 deletions

diff --git a/CHANGELOG b/CHANGELOG
index a47ec511452..11141b38a0e 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -31,6 +31,7 @@ v 8.13.0 (unreleased)
   - Take filters in account in issuable counters. !6496
   - Use custom Ruby images to test builds (registry.dev.gitlab.org/gitlab/gitlab-build-images:*)
   - Append issue template to existing description !6149 (Joseph Frazier)
+  - Trending projects now only show public projects and the list of projects is cached for a day
   - Revoke button in Applications Settings underlines on hover.
   - Add missing values to linter !6276 (Katarzyna Kobierska Ula Budziszewska)
   - Fix Long commit messages overflow viewport in file tree
diff --git a/app/controllers/explore/projects_controller.rb b/app/controllers/explore/projects_controller.rb
index 88a0c18180b..38e5943eb76 100644
--- a/app/controllers/explore/projects_controller.rb
+++ b/app/controllers/explore/projects_controller.rb
@@ -21,7 +21,7 @@ class Explore::ProjectsController < Explore::ApplicationController
   end
 
   def trending
-    @projects = TrendingProjectsFinder.new.execute(current_user)
+    @projects = TrendingProjectsFinder.new.execute
     @projects = filter_projects(@projects)
     @projects = @projects.page(params[:page])
 
diff --git a/app/finders/trending_projects_finder.rb b/app/finders/trending_projects_finder.rb
index 81a12403801..48c98731cbc 100644
--- a/app/finders/trending_projects_finder.rb
+++ b/app/finders/trending_projects_finder.rb
@@ -1,6 +1,11 @@
+# Finder for retrieving public trending projects in a given time range.
 class TrendingProjectsFinder
-  def execute(current_user, start_date = 1.month.ago)
-    projects_for(current_user).trending(start_date)
+  # current_user - The currently logged in User, if any.
+  # last_months - The number of months to limit the trending data to.
+  def execute(months_limit = 1)
+    Rails.cache.fetch(cache_key_for(months_limit), expires_in: 1.day) do
+      Project.public_only.trending(months_limit.months.ago)
+    end
   end
 
   private
@@ -8,4 +13,8 @@ class TrendingProjectsFinder
   def projects_for(current_user)
     ProjectsFinder.new.execute(current_user)
   end
+
+  def cache_key_for(months)
+    "trending_projects/#{months}"
+  end
 end
diff --git a/spec/finders/trending_projects_finder_spec.rb b/spec/finders/trending_projects_finder_spec.rb
index a49cbfd5160..cfe15b9defa 100644
--- a/spec/finders/trending_projects_finder_spec.rb
+++ b/spec/finders/trending_projects_finder_spec.rb
@@ -1,39 +1,48 @@
 require 'spec_helper'
 
 describe TrendingProjectsFinder do
-  let(:user) { build(:user) }
+  let(:user) { create(:user) }
+  let(:public_project1) { create(:empty_project, :public) }
+  let(:public_project2) { create(:empty_project, :public) }
+  let(:private_project) { create(:empty_project, :private) }
+  let(:internal_project) { create(:empty_project, :internal) }
+
+  before do
+    3.times do
+      create(:note_on_commit, project: public_project1)
+    end
 
-  describe '#execute' do
-    describe 'without an explicit start date' do
-      subject { described_class.new }
+    2.times do
+      create(:note_on_commit, project: public_project2, created_at: 5.weeks.ago)
+    end
 
-      it 'returns the trending projects' do
-        relation = double(:ar_relation)
+    create(:note_on_commit, project: private_project)
+    create(:note_on_commit, project: internal_project)
+  end
 
-        allow(subject).to receive(:projects_for)
-          .with(user)
-          .and_return(relation)
+  describe '#execute', caching: true do
+    context 'without an explicit time range' do
+      it 'returns public trending projects' do
+        projects = described_class.new.execute
 
-        allow(relation).to receive(:trending)
-          .with(an_instance_of(ActiveSupport::TimeWithZone))
+        expect(projects).to eq([public_project1])
       end
     end
 
-    describe 'with an explicit start date' do
-      let(:date) { 2.months.ago }
+    context 'with an explicit time range' do
+      it 'returns public trending projects' do
+        projects = described_class.new.execute(2)
 
-      subject { described_class.new }
+        expect(projects).to eq([public_project1, public_project2])
+      end
+    end
 
-      it 'returns the trending projects' do
-        relation = double(:ar_relation)
+    it 'caches the list of projects' do
+      projects = described_class.new
 
-        allow(subject).to receive(:projects_for)
-          .with(user)
-          .and_return(relation)
+      expect(Project).to receive(:trending).once
 
-        allow(relation).to receive(:trending)
-          .with(date)
-      end
+      2.times { projects.execute }
     end
   end
 end