Improve performance of audit events CSV export

For search UI, the audit events scope is limit by pagination. This does
not exhibit the same problem with CSV export where scope does not
have set limit (we use file size limit as the stopping mechanism).

Preloading associated records, hence, requires to be performed in
batches to avoid overloading memory. BatchLoader cache was designed to
hold data for one web request, which in our case might yield significant
amount of user records. To keep the memory growth under control, we're
clearing up the cached data after each iteration (i.e. after 1K
records).

A fix for `MissingAttribute` error when calling `select` without
`details` field is also introduced. This is caused by the call back
method to initialize `details` after AR model loaded from database. This
change allows the following usage: `AuditEvent.select(:id).first`.
Co-authored-by: Tan Le <tle@gitlab.com>

app/models/audit_event.rb

@@ -4,6 +4,7 @@ class AuditEvent < ApplicationRecord
   include CreatedAtFilterable
   include IgnorableColumns
   include BulkInsertSafe
+  include EachBatch
 
   PARALLEL_PERSISTENCE_COLUMNS = [
     :author_name,
@@ -54,7 +55,9 @@ class AuditEvent < ApplicationRecord
   end
 
   def initialize_details
-    self.details = {} if details.nil?
+    return unless self.has_attribute?(:details)
+
+    self.details = {} if details&.nil?
   end
 
   def author_name

ee/app/services/audit_events/export_csv_service.rb

@@ -20,7 +20,7 @@ module AuditEvents
 
     def data
       events = AuditLogFinder.new(finder_params).execute
-      Gitlab::Audit::Events::Preloader.preload!(events)
+      Gitlab::Audit::Events::Preloader.new(events)
     end
 
     def finder_params

ee/changelogs/unreleased/266976-audit-event-export-improvement-for-csv-export.yml

+---
+title: Improve audit events preloading logic for CSV export
+merge_request: 44958
+author:
+type: performance

ee/lib/audit/details.rb

@@ -109,7 +109,7 @@ module Audit
     end
 
     def detail_value
-      @details.each_value.first
+      String(@details.each_value.first)
     end
 
     def oauth_label

ee/lib/gitlab/audit/events/preloader.rb

@@ -12,6 +12,25 @@ module Gitlab
             end
           end
         end
+
+        def initialize(audit_events)
+          @audit_events = audit_events
+        end
+
+        def find_each(&block)
+          @audit_events.each_batch(column: :created_at) do |relation|
+            relation.each do |audit_event|
+              audit_event.lazy_author
+              audit_event.lazy_entity
+            end
+
+            relation.each do |audit_event|
+              yield(audit_event)
+            end
+
+            BatchLoader::Executor.clear_current
+          end
+        end
       end
     end
   end

ee/spec/lib/gitlab/audit/events/preloader_spec.rb

@@ -3,11 +3,10 @@
 require 'spec_helper'
 
 RSpec.describe Gitlab::Audit::Events::Preloader do
-  describe '.preload!' do
-    let_it_be(:audit_events) { create_list(:audit_event, 2) }
-
-    let(:audit_events_relation) { AuditEvent.where(id: audit_events.map(&:id)) }
+  let_it_be(:audit_events) { create_list(:audit_event, 2) }
+  let(:audit_events_relation) { AuditEvent.where(id: audit_events.map(&:id)) }
 
+  describe '.preload!' do
     subject { described_class.preload!(audit_events_relation) }
 
     it 'returns an ActiveRecord::Relation' do
@@ -15,12 +14,6 @@ RSpec.describe Gitlab::Audit::Events::Preloader do
     end
 
     it 'preloads associated records' do
-      log = ActiveRecord::QueryRecorder.new do
-        subject.map do |event|
-          [event.author_name, event.lazy_entity.name]
-        end
-      end
-
       # Expected queries when requesting for AuditEvent with associated records
       #
       # 1. On the audit_events table
@@ -30,7 +23,39 @@ RSpec.describe Gitlab::Audit::Events::Preloader do
       # 3. On the users table for entity name
       #    SELECT "users".* FROM "users" WHERE "users"."id" IN (2, 4)
       #
-      expect(log.count).to eq(3)
+      expect do
+        subject.map do |event|
+          [event.author_name, event.lazy_entity.name]
+        end
+      end.not_to exceed_query_limit(3)
+    end
+  end
+
+  describe '#find_each' do
+    let(:preloader) { described_class.new(audit_events_relation) }
+
+    it 'yields a list audit events' do
+      expect { |b| preloader.find_each(&b) }.to yield_successive_args(*audit_events)
+    end
+
+    it 'loads audit events in batches with preloaded associated records' do
+      # Expected queries when requesting for AuditEvent with associated records
+      #
+      # 1. Get the start of created_at value in 1k batch
+      #    SELECT "audit_events"."created_at" FROM "audit_events" WHERE "audit_events"."id" IN (1, 2) ORDER BY "audit_events"."created_at" ASC LIMIT 1
+      # 2. Get the end of created_at value in 1k batch
+      #    SELECT "audit_events"."created_at" FROM "audit_events" WHERE "audit_events"."id" IN (1, 2) AND "audit_events"."created_at" >= '2020-10-15 04:51:06.392709' ORDER BY "audit_events"."created_at" ASC LIMIT 1 OFFSET 1000
+      # 3. Get the audit_events in 1k batch
+      #    SELECT "audit_events".* FROM "audit_events" WHERE "audit_events"."id" IN (1, 2) AND "audit_events"."created_at" >= '2020-10-15 04:51:06.392709'
+      # 4. On the users table for author_name
+      #    SELECT "users"."id", "users"."name", "users"."username" FROM "users" WHERE "users"."id" IN (1, 3) ORDER BY "users"."id" ASC LIMIT 1000
+      # 5. On the users table for entity name
+      #    SELECT "users".* FROM "users" WHERE "users"."id" IN (2, 4) ORDER BY "users"."id" ASC LIMIT 1000
+      expect do
+        preloader.find_each do |event|
+          [event.author_name, event.lazy_entity.name]
+        end
+      end.not_to exceed_query_limit(5)
     end
   end
 end