Add feature flag and comments on hash structure

lib/gitlab/import_export/fast_hash_serializer.rb

@@ -79,6 +79,11 @@ module Gitlab
         @tree = tree
       end
 
+      # With the usage of `JSONBatchRelation`, it returns partially
+      # serialized hash which is not easily accessible.
+      # It means you can only manipulate and replace top-level objects.
+      # All future mutations of the hash (such as `fix_project_tree`)
+      # should be aware of that.
       def execute
         simple_serialize.merge(serialize_includes)
       end
@@ -131,7 +136,12 @@ module Gitlab
         data = []
 
         record.in_batches(of: @batch_size) do |batch| # rubocop:disable Cop/InBatches
-          data.append(JSONBatchRelation.new(batch, options, preloads[key]).tap(&:raw_json))
+          if Feature.enabled?(:export_fast_serialize_with_raw_json, default_enabled: true)
+            data.append(JSONBatchRelation.new(batch, options, preloads[key]).tap(&:raw_json))
+          else
+            batch = batch.preload(preloads[key]) if preloads&.key?(key)
+            data += batch.as_json(options)
+          end
         end
 
         data

lib/gitlab/import_export/project_tree_saver.rb

@@ -31,6 +31,8 @@ module Gitlab
 
       private
 
+      # Aware that the resulting hash needs to be pure-hash and
+      # does not include any AR objects anymore, only objects that run `.to_json`
       def fix_project_tree(project_tree)
         if @params[:description].present?
           project_tree['description'] = @params[:description]

spec/lib/gitlab/import_export/fast_hash_serializer_spec.rb

 require 'spec_helper'
 
 describe Gitlab::ImportExport::FastHashSerializer do
+  # FastHashSerializer#execute generates the hash which is not easily accessible
+  # and includes `JSONBatchRelation` items which are serialized at this point.
+  # Wrapping the result into JSON generating/parsing is for making
+  # the testing more convenient. Doing this, we can check that
+  # all items are properly serialized while traversing the simple hash.
   subject { JSON.parse(JSON.generate(described_class.new(project, tree).execute)) }
 
   let!(:project) { setup_project }