Revamp trending projects query

This changes the query to use a COUNT nested in an INNER JOIN, instead
of a COUNT plus a GROUP BY. There are two reasons for this:

1. Using a COUNT in an INNER JOIN can be quite a bit faster.
2. The use of a GROUP BY means that method calls such as "any?"
   (and everything else that calls "count") operate on a Hash that
   counts the amount of notes on a per project basis, instead of just
   counting the total amount of projects.

The query has been moved into Project.trending as its logic is simple
enough. As a result of this testing the TrendingProjectsFinder class
simply involves testing if the right methods are called, removing the
need for setting up database records.

app/finders/trending_projects_finder.rb

 class TrendingProjectsFinder
-  def execute(current_user, start_date = nil)
-    start_date ||= Date.today - 1.month
-
-    projects = projects_for(current_user)
-
-    # Determine trending projects based on comments count
-    # for period of time - ex. month
-    projects.joins(:notes).where('notes.created_at >= ?', start_date).
-      group("projects.id").reorder("count(notes.id) DESC")
+  def execute(current_user, start_date = 1.month.ago)
+    projects_for(current_user).trending(start_date)
   end
 
   private

app/models/project.rb

@@ -260,6 +260,20 @@ class Project < ActiveRecord::Base
       name_pattern = Gitlab::Regex::NAMESPACE_REGEX_STR
       %r{(?<project>#{name_pattern}/#{name_pattern})}
     end
+
+    def trending(since = 1.month.ago)
+      # By counting in the JOIN we don't expose the GROUP BY to the outer query.
+      # This means that calls such as "any?" and "count" just return a number of
+      # the total count, instead of the counts grouped per project as a Hash.
+      join_body = "INNER JOIN (
+        SELECT project_id, COUNT(*) AS amount
+        FROM notes
+        WHERE created_at >= #{sanitize(since)}
+        GROUP BY project_id
+      ) join_note_counts ON projects.id = join_note_counts.project_id"
+
+      joins(join_body).reorder('join_note_counts.amount DESC')
+    end
   end
 
   def team

spec/finders/trending_projects_finder_spec.rb

 require 'spec_helper'
 
 describe TrendingProjectsFinder do
-  let(:user)  { create(:user) }
-  let(:group) { create(:group) }
-
-  let(:project1) { create(:empty_project, :public, group: group) }
-  let(:project2) { create(:empty_project, :public, group: group) }
-
-  before do
-    2.times do
-      create(:note_on_commit, project: project1)
-    end
-
-    create(:note_on_commit, project: project2)
-  end
+  let(:user) { build(:user) }
 
   describe '#execute' do
     describe 'without an explicit start date' do
-      subject { described_class.new.execute(user).to_a }
+      subject { described_class.new }
 
-      it 'sorts Projects by the amount of notes in descending order' do
-        expect(subject).to eq([project1, project2])
+      it 'returns the trending projects' do
+        relation = double(:ar_relation)
+
+        allow(subject).to receive(:projects_for)
+          .with(user)
+          .and_return(relation)
+
+        allow(relation).to receive(:trending)
+          .with(an_instance_of(ActiveSupport::TimeWithZone))
       end
     end
 
     describe 'with an explicit start date' do
       let(:date) { 2.months.ago }
 
-      subject { described_class.new.execute(user, date).to_a }
+      subject { described_class.new }
 
-      before do
-        2.times do
-          create(:note_on_commit, project: project2, created_at: date)
-        end
-      end
+      it 'returns the trending projects' do
+        relation = double(:ar_relation)
+
+        allow(subject).to receive(:projects_for)
+          .with(user)
+          .and_return(relation)
 
-      it 'sorts Projects by the amount of notes in descending order' do
-        expect(subject).to eq([project2, project1])
+        allow(relation).to receive(:trending)
+          .with(date)
       end
     end
   end

spec/models/project_spec.rb

@@ -423,4 +423,42 @@ describe Project do
     it { expect(project.gitlab_ci?).to be_truthy }
     it { expect(project.gitlab_ci_project).to be_a(Ci::Project) }
   end
+
+  describe '.trending' do
+    let(:group)    { create(:group) }
+    let(:project1) { create(:empty_project, :public, group: group) }
+    let(:project2) { create(:empty_project, :public, group: group) }
+
+    before do
+      2.times do
+        create(:note_on_commit, project: project1)
+      end
+
+      create(:note_on_commit, project: project2)
+    end
+
+    describe 'without an explicit start date' do
+      subject { described_class.trending.to_a }
+
+      it 'sorts Projects by the amount of notes in descending order' do
+        expect(subject).to eq([project1, project2])
+      end
+    end
+
+    describe 'with an explicit start date' do
+      let(:date) { 2.months.ago }
+
+      subject { described_class.trending(date).to_a }
+
+      before do
+        2.times do
+          create(:note_on_commit, project: project2, created_at: date)
+        end
+      end
+
+      it 'sorts Projects by the amount of notes in descending order' do
+        expect(subject).to eq([project2, project1])
+      end
+    end
+  end
 end