Use Atom update times of the first eventevents-performance

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

7 files changed, 5 insertions, 34 deletions

diff --git a/app/models/event.rb b/app/models/event.rb
index 8dfc1d04709..4be23a1cf72 100644
--- a/app/models/event.rb
+++ b/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
diff --git a/app/models/project.rb b/app/models/project.rb
index 63ecf5b7105..4bd51449c25 100644
--- a/app/models/project.rb
+++ b/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
diff --git a/app/views/dashboard/projects/index.atom.builder b/app/views/dashboard/projects/index.atom.builder
index 2e2712c5146..d4daf07c6c0 100644
--- a/app/views/dashboard/projects/index.atom.builder
+++ b/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)
diff --git a/app/views/groups/show.atom.builder b/app/views/groups/show.atom.builder
index 5cc0f5e1d2e..c66b82bb484 100644
--- a/app/views/groups/show.atom.builder
+++ b/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)
diff --git a/app/views/projects/show.atom.builder b/app/views/projects/show.atom.builder
index d6762219108..2468509242a 100644
--- a/app/views/projects/show.atom.builder
+++ b/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)
diff --git a/app/views/users/show.atom.builder b/app/views/users/show.atom.builder
index 114d1e7a379..e9e466c6350 100644
--- a/app/views/users/show.atom.builder
+++ b/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)
diff --git a/spec/models/event_spec.rb b/spec/models/event_spec.rb
index 071582b0282..ec2a923f91b 100644
--- a/spec/models/event_spec.rb
+++ b/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) }