Reorder exported relations by primary_key

changelogs/unreleased/georgekoltsov-fix-relations-order-on-export.yml

+---
+title: Reorder exported relations by primary_key when using Project Export
+merge_request: 27117
+author:
+type: fixed

lib/gitlab/import_export/fast_hash_serializer.rb

@@ -136,6 +136,12 @@ module Gitlab
         data = []
 
         record.in_batches(of: @batch_size) do |batch| # rubocop:disable Cop/InBatches
+          # order each batch by it's primary key to ensure
+          # consistent and predictable ordering of each exported relation
+          # as additional `WHERE` clauses can impact the order in which data is being
+          # returned by database when no `ORDER` is specified
+          batch = batch.reorder(batch.klass.primary_key)
+
           if Feature.enabled?(:export_fast_serialize_with_raw_json, default_enabled: true)
             data.append(JSONBatchRelation.new(batch, options, preloads[key]).tap(&:raw_json))
           else

spec/lib/gitlab/import_export/fast_hash_serializer_spec.rb

@@ -215,6 +215,14 @@ describe Gitlab::ImportExport::FastHashSerializer do
     expect(subject['boards'].first['lists']).not_to be_empty
   end
 
+  context 'relation ordering' do
+    it 'orders exported pipelines by primary key' do
+      expected_order = project.ci_pipelines.reorder(:id).ids
+
+      expect(subject['ci_pipelines'].pluck('id')).to eq(expected_order)
+    end
+  end
+
   def setup_project
     release = create(:release)
     group = create(:group)
@@ -246,6 +254,8 @@ describe Gitlab::ImportExport::FastHashSerializer do
     ci_build.pipeline.update(project: project)
     create(:commit_status, project: project, pipeline: ci_build.pipeline)
 
+    create_list(:ci_pipeline, 5, :success, project: project)
+
     create(:milestone, project: project)
     create(:discussion_note, noteable: issue, project: project)
     create(:note, noteable: merge_request, project: project)