Use Atom update times of the first event

By simply loading the first event from the already sorted set we save
ourselves extra (slow) queries just to get the latest update timestamp.
This removes the need for Event.latest_update_time and significantly
reduces the time needed to build an Atom feed.

Fixes gitlab-org/gitlab-ce#12415

app/models/event.rb

@@ -49,7 +49,7 @@ class Event < ActiveRecord::Base
   scope :code_push, -> { where(action: PUSHED) }
 
   scope :in_projects, ->(projects) do
-    where(project_id: projects.reorder(nil).id_only).recent
+    where(project_id: projects.select(:id).reorder(nil)).recent
   end
 
   scope :with_associations, -> { includes(project: :namespace) }
@@ -68,12 +68,6 @@ class Event < ActiveRecord::Base
             [Event::CREATED, Event::CLOSED, Event::MERGED])
     end
 
-    def latest_update_time
-      row = select(:updated_at, :project_id).reorder(id: :desc).take
-
-      row ? row.updated_at : nil
-    end
-
     def limit_recent(limit = 20, offset = nil)
       recent.limit(limit).offset(offset)
     end

app/models/project.rb

@@ -215,8 +215,6 @@ class Project < ActiveRecord::Base
   scope :public_and_internal_only, -> { where(visibility_level: Project.public_and_internal_levels) }
   scope :non_archived, -> { where(archived: false) }
 
-  scope :id_only, -> { select(:id) }
-
   state_machine :import_status, initial: :none do
     event :import_start do
       transition [:none, :finished] => :started

app/views/dashboard/projects/index.atom.builder

@@ -4,7 +4,7 @@ xml.feed "xmlns" => "http://www.w3.org/2005/Atom", "xmlns:media" => "http://sear
   xml.link    href: dashboard_projects_url(format: :atom, private_token: current_user.try(:private_token)), rel: "self", type: "application/atom+xml"
   xml.link    href: dashboard_projects_url, rel: "alternate", type: "text/html"
   xml.id      dashboard_projects_url
-  xml.updated @events.latest_update_time.xmlschema if @events.any?
+  xml.updated @events[0].updated_at.xmlschema if @events[0]
 
   @events.each do |event|
     event_to_atom(xml, event)

app/views/groups/show.atom.builder

@@ -4,7 +4,7 @@ xml.feed "xmlns" => "http://www.w3.org/2005/Atom", "xmlns:media" => "http://sear
   xml.link    href: group_url(@group, format: :atom, private_token: current_user.try(:private_token)), rel: "self", type: "application/atom+xml"
   xml.link    href: group_url(@group), rel: "alternate", type: "text/html"
   xml.id      group_url(@group)
-  xml.updated @events.latest_update_time.xmlschema if @events.any?
+  xml.updated @events[0].updated_at.xmlschema if @events[0]
 
   @events.each do |event|
     event_to_atom(xml, event)

app/views/projects/show.atom.builder

@@ -4,7 +4,7 @@ xml.feed "xmlns" => "http://www.w3.org/2005/Atom", "xmlns:media" => "http://sear
   xml.link    href: namespace_project_url(@project.namespace, @project, format: :atom, private_token: current_user.try(:private_token)), rel: "self", type: "application/atom+xml"
   xml.link    href: namespace_project_url(@project.namespace, @project), rel: "alternate", type: "text/html"
   xml.id      namespace_project_url(@project.namespace, @project)
-  xml.updated @events.latest_update_time.xmlschema if @events.any?
+  xml.updated @events[0].updated_at.xmlschema if @events[0?
 
   @events.each do |event|
     event_to_atom(xml, event)

app/views/users/show.atom.builder

@@ -4,7 +4,7 @@ xml.feed "xmlns" => "http://www.w3.org/2005/Atom", "xmlns:media" => "http://sear
   xml.link    href: user_url(@user, :atom), rel: "self", type: "application/atom+xml"
   xml.link    href: user_url(@user), rel: "alternate", type: "text/html"
   xml.id      user_url(@user)
-  xml.updated @events.latest_update_time.xmlschema if @events.any?
+  xml.updated @events[0].updated_at.xmlschema if @events[0]
 
   @events.each do |event|
     event_to_atom(xml, event)

spec/models/event_spec.rb

@@ -65,27 +65,6 @@ describe Event, models: true do
     it { expect(@event.author).to eq(@user) }
   end
 
-  describe '.latest_update_time' do
-    describe 'when events are present' do
-      let(:time) { Time.utc(2015, 1, 1) }
-
-      before do
-        create(:closed_issue_event, updated_at: time)
-        create(:closed_issue_event, updated_at: time + 5)
-      end
-
-      it 'returns the latest update time' do
-        expect(Event.latest_update_time).to eq(time + 5)
-      end
-    end
-
-    describe 'when no events exist' do
-      it 'returns nil' do
-        expect(Event.latest_update_time).to be_nil
-      end
-    end
-  end
-
   describe '.limit_recent' do
     let!(:event1) { create(:closed_issue_event) }
     let!(:event2) { create(:closed_issue_event) }