Use #in_batches in Project Export StreamingSerializer

lib/gitlab/import_export/json/streaming_serializer.rb

@@ -69,10 +69,18 @@ module Gitlab
             key_preloads = preloads&.dig(key)
             records = records.preload(key_preloads) if key_preloads
 
-            records.find_each(batch_size: batch_size) do |record|
+            records.in_batches(of: batch_size) do |batch| # rubocop:disable Cop/InBatches
+              # order each batch by its 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)
+
+              batch.each do |record|
                 items << Raw.new(record.to_json(options))
               end
             end
+          end
 
           json_writer.write_relation_array(@exportable_path, key, enumerator)
         end

spec/lib/gitlab/import_export/json/streaming_serializer_spec.rb

@@ -61,6 +61,20 @@ describe Gitlab::ImportExport::JSON::StreamingSerializer do
 
         subject.execute
       end
+
+      context 'relation ordering' do
+        before do
+          create_list(:issue, 5, project: exportable)
+        end
+
+        it 'orders exported issues by primary key' do
+          expected_issues = exportable.issues.reorder(:id).map(&:to_json)
+
+          expect(json_writer).to receive(:write_relation_array).with(exportable_path, :issues, expected_issues)
+
+          subject.execute
+        end
+      end
     end
 
     context 'with single relation' do