Introduce bulk insert of weight changes for an array of issues

Create FKs to issues and users as part of `references`
Use bulk insert in weight events tracking

db/migrate/20191210211253_create_resource_weight_event.rb

 # frozen_string_literal: true
 
 class CreateResourceWeightEvent < ActiveRecord::Migration[5.2]
-  include Gitlab::Database::MigrationHelpers
-
   DOWNTIME = false
 
-  disable_ddl_transaction!
-
-  def up
+  def change
     create_table :resource_weight_events do |t|
-      t.integer :user_id, null: false
-      t.integer :issue_id, null: false
+      t.references :user, null: false, foreign_key: { on_delete: :cascade },
+                   index: { name: 'index_resource_weight_events_on_user_id' }
+      t.references :issue, null: false, foreign_key: { on_delete: :cascade },
+                   index: false
       t.integer :weight
       t.datetime_with_timezone :created_at, null: false
 
-      t.index [:user_id], name: 'index_resource_weight_events_on_user_id'
       t.index [:issue_id, :weight], name: 'index_resource_weight_events_on_issue_id_and_weight'
     end
-
-    add_concurrent_foreign_key :resource_weight_events, :users, column: :user_id, on_delete: :cascade
-    add_concurrent_foreign_key :resource_weight_events, :issues, column: :issue_id, on_delete: :cascade
-  end
-
-  def down
-    drop_table :resource_weight_events
   end
 end

db/schema.rb

@@ -3606,8 +3606,8 @@ ActiveRecord::Schema.define(version: 2019_12_18_225624) do
   end
 
   create_table "resource_weight_events", force: :cascade do |t|
-    t.integer "user_id", null: false
-    t.integer "issue_id", null: false
+    t.bigint "user_id", null: false
+    t.bigint "issue_id", null: false
     t.integer "weight"
     t.datetime_with_timezone "created_at", null: false
     t.index ["issue_id", "weight"], name: "index_resource_weight_events_on_issue_id_and_weight"
@@ -4754,8 +4754,8 @@ ActiveRecord::Schema.define(version: 2019_12_18_225624) do
   add_foreign_key "resource_label_events", "labels", on_delete: :nullify
   add_foreign_key "resource_label_events", "merge_requests", on_delete: :cascade
   add_foreign_key "resource_label_events", "users", on_delete: :nullify
-  add_foreign_key "resource_weight_events", "issues", name: "fk_5eb5cb92a1", on_delete: :cascade
-  add_foreign_key "resource_weight_events", "users", name: "fk_bfc406b47c", on_delete: :cascade
+  add_foreign_key "resource_weight_events", "issues", on_delete: :cascade
+  add_foreign_key "resource_weight_events", "users", on_delete: :cascade
   add_foreign_key "reviews", "merge_requests", on_delete: :cascade
   add_foreign_key "reviews", "projects", on_delete: :cascade
   add_foreign_key "reviews", "users", column: "author_id", on_delete: :nullify

ee/app/services/ee/issuable/common_system_notes_service.rb

@@ -29,7 +29,7 @@ module EE
       end
 
       def handle_weight_change_note
-        if issuable.previous_changes.include?('weight')
+        if !issuable.is_a?(Epic) && issuable.previous_changes.include?('weight')
           note = create_weight_change_note
 
           track_weight_change(note)
@@ -43,11 +43,11 @@ module EE
       def track_weight_change(note)
         return unless weight_changes_tracking_enabled?
 
-        EE::ResourceEvents::ChangeWeightService.new(issuable, current_user, note.created_at).execute
+        EE::ResourceEvents::ChangeWeightService.new([issuable], current_user, note.created_at).execute
       end
 
       def weight_changes_tracking_enabled?
-        ::Feature.enabled?(:track_resource_weight_change_events, issuable.project)
+        ::Feature.enabled?(:track_issue_weight_change_events, issuable.project)
       end
     end
   end

ee/app/services/ee/resource_events/change_weight_service.rb

@@ -3,32 +3,36 @@
 module EE
   module ResourceEvents
     class ChangeWeightService
-      attr_reader :resource, :user, :event_created_at
+      attr_reader :resources, :user, :event_created_at
 
-      def initialize(resource, user, created_at)
-        @resource = resource
+      def initialize(resources, user, created_at)
+        @resources = resources
         @user = user
         @event_created_at = created_at
       end
 
       def execute
-        create_event_by_issue if first_weight_event?
-
-        ResourceWeightEvent
-          .new(user: user, issue: resource, weight: resource.weight, created_at: event_created_at)
-          .save
+        ::Gitlab::Database.bulk_insert(ResourceWeightEvent.table_name, resource_weight_changes) unless resource_weight_changes.empty?
       end
 
       private
 
-      def first_weight_event?
-        ResourceWeightEvent.by_issue(resource).none?
+      def resource_weight_changes
+        @weight_changes ||= resources.map do |resource|
+          changes = []
+          base_data = { user_id: user.id, issue_id: resource.id }
+
+          changes << base_data.merge({ weight: previous_weight(resource), created_at: resource.updated_at }) if first_weight_event?(resource)
+          changes << base_data.merge({ weight: resource.weight, created_at: event_created_at })
+        end.flatten
+      end
+
+      def previous_weight(resource)
+        resource.previous_changes['weight']&.first
       end
 
-      def create_event_by_issue
-        ResourceWeightEvent
-          .new(user: user, issue: resource, weight: resource.weight, created_at: resource.updated_at)
-          .save
+      def first_weight_event?(resource)
+        previous_weight(resource) && ResourceWeightEvent.by_issue(resource).none?
       end
     end
   end

ee/spec/services/ee/issuable/common_system_notes_service_spec.rb

@@ -47,7 +47,7 @@ describe Issuable::CommonSystemNotesService do
 
     context 'when resource weight event tracking is enabled' do
       before do
-        stub_feature_flags(track_resource_weight_change_events: true)
+        stub_feature_flags(track_issue_weight_change_events: true)
       end
 
       it 'creates a resource weight event' do
@@ -63,7 +63,7 @@ describe Issuable::CommonSystemNotesService do
 
     context 'when resource weight event tracking is disabled' do
       before do
-        stub_feature_flags(track_resource_weight_change_events: false)
+        stub_feature_flags(track_issue_weight_change_events: false)
       end
 
       it 'does not created a resource weight event' do

ee/spec/services/ee/resource_events/change_weight_service_spec.rb

@@ -8,7 +8,7 @@ RSpec.describe EE::ResourceEvents::ChangeWeightService do
   let(:issue) { create(:issue, weight: 3) }
   let(:created_at_time) { Time.new(2019, 1, 1, 12, 30, 48) }
 
-  subject { described_class.new(issue, user, created_at_time).execute }
+  subject { described_class.new([issue], user, created_at_time).execute }
 
   before do
     ResourceWeightEvent.new(issue: issue, user: user).save!
@@ -35,6 +35,7 @@ RSpec.describe EE::ResourceEvents::ChangeWeightService do
   context 'when there is no existing weight event record' do
     before do
       ResourceWeightEvent.delete_all
+      issue.update(weight: 5)
     end
 
     it 'creates the expected event records' do
@@ -44,7 +45,7 @@ RSpec.describe EE::ResourceEvents::ChangeWeightService do
       expect_event_record(record, weight: 3, created_at: issue.reload.updated_at)
 
       record = ResourceWeightEvent.last
-      expect_event_record(record, weight: 3, created_at: created_at_time)
+      expect_event_record(record, weight: 5, created_at: created_at_time)
     end
   end
 
@@ -54,4 +55,25 @@ RSpec.describe EE::ResourceEvents::ChangeWeightService do
     expect(record.weight).to eq(weight)
     expect(record.created_at).to eq(created_at)
   end
+
+  describe 'bulk issue weight updates' do
+    let(:issues) { create_list(:issue, 3, weight: 1) }
+
+    before do
+      issues.each { |issue| issue.update!(weight: 3) }
+    end
+
+    it 'bulk insert weight changes' do
+      expect do
+        described_class.new(issues, user, created_at_time).execute
+      end.to change { ResourceWeightEvent.count }.by(6)
+    end
+
+    it 'calls first_weight_event? once per resource' do
+      service = described_class.new(issues, user, created_at_time)
+      allow(service).to receive(:first_weight_event?).exactly(3).times
+
+      service.execute
+    end
+  end
 end