Merge branch 'kamil-improve-import-export' into 'master'

Improve performance and (memory usage (a little)) of project export

See merge request gitlab-org/gitlab-ce!32423

10 files changed, 547 insertions, 19 deletions

diff --git a/changelogs/unreleased/kamil-improve-import-export.yml b/changelogs/unreleased/kamil-improve-import-export.yml
new file mode 100644
index 00000000000..9d485e0df2d
--- /dev/null
+++ b/changelogs/unreleased/kamil-improve-import-export.yml
@@ -0,0 +1,5 @@
+---
+title: Reduce N+1 when doing project export
+merge_request: 32423
+author:
+type: performance
diff --git a/lib/gitlab/import_export/attributes_finder.rb b/lib/gitlab/import_export/attributes_finder.rb
index 13883ca7f3d..28d48ce6dfe 100644
--- a/lib/gitlab/import_export/attributes_finder.rb
+++ b/lib/gitlab/import_export/attributes_finder.rb
@@ -8,6 +8,7 @@ module Gitlab
         @included_attributes = config[:included_attributes] || {}
         @excluded_attributes = config[:excluded_attributes] || {}
         @methods = config[:methods] || {}
+        @preloads = config[:preloads] || {}
       end
 
       def find_root(model_key)
@@ -29,10 +30,26 @@ module Gitlab
           only: @included_attributes[model_key],
           except: @excluded_attributes[model_key],
           methods: @methods[model_key],
-          include: resolve_model_tree(model_tree)
+          include: resolve_model_tree(model_tree),
+          preload: resolve_preloads(model_key, model_tree)
         }.compact
       end
 
+      def resolve_preloads(model_key, model_tree)
+        model_tree
+          .map { |submodel_key, submodel_tree| resolve_preload(model_key, submodel_key, submodel_tree) }
+          .compact
+          .to_h
+          .deep_merge(@preloads[model_key].to_h)
+          .presence
+      end
+
+      def resolve_preload(parent_model_key, model_key, model_tree)
+        return if @methods[parent_model_key]&.include?(model_key)
+
+        [model_key, resolve_preloads(model_key, model_tree)]
+      end
+
       def resolve_model_tree(model_tree)
         return unless model_tree
 
diff --git a/lib/gitlab/import_export/fast_hash_serializer.rb b/lib/gitlab/import_export/fast_hash_serializer.rb
new file mode 100644
index 00000000000..a6ab4f3a3d9
--- /dev/null
+++ b/lib/gitlab/import_export/fast_hash_serializer.rb
@@ -0,0 +1,108 @@
+# frozen_string_literal: true
+
+# ActiveModel::Serialization (https://github.com/rails/rails/blob/v5.0.7/activemodel/lib/active_model/serialization.rb#L184)
+# is simple in that it recursively calls `as_json` on each object to
+# serialize everything. However, for a model like a Project, this can
+# generate a query for every single association, which can add up to tens
+# of thousands of queries and lead to memory bloat.
+#
+# To improve this, we can do several things:
+
+# 1. Use the option tree in http://api.rubyonrails.org/classes/ActiveModel/Serializers/JSON.html
+#    to generate the necessary preload clauses.
+#
+# 2. We observe that a single project has many issues, merge requests,
+#    etc. Instead of serializing everything at once, which could lead to
+#    database timeouts and high memory usage, we take each top-level
+#    association and serialize the data in batches.
+#
+#  For example, we serialize the first 100 issues and preload all of
+#  their associated events, notes, etc. before moving onto the next
+#  batch. When we're done, we serialize merge requests in the same way.
+#  We repeat this pattern for the remaining associations specified in
+#  import_export.yml.
+module Gitlab
+  module ImportExport
+    class FastHashSerializer
+      attr_reader :subject, :tree
+
+      BATCH_SIZE = 100
+
+      def initialize(subject, tree, batch_size: BATCH_SIZE)
+        @subject = subject
+        @batch_size = batch_size
+        @tree = tree
+      end
+
+      # Serializes the subject into a Hash for the given option tree
+      # (e.g. Project#as_json)
+      def execute
+        simple_serialize.merge(serialize_includes)
+      end
+
+      private
+
+      def simple_serialize
+        subject.as_json(
+          tree.merge(include: nil, preloads: nil))
+      end
+
+      def serialize_includes
+        return {} unless includes
+
+        includes
+          .map(&method(:serialize_include_definition))
+          .compact
+          .to_h
+      end
+
+      # definition:
+      # { labels: { includes: ... } }
+      def serialize_include_definition(definition)
+        raise ArgumentError, 'definition needs to be Hash' unless definition.is_a?(Hash)
+        raise ArgumentError, 'definition needs to have exactly one Hash element' unless definition.one?
+
+        key = definition.first.first
+        options = definition.first.second
+
+        record = subject.public_send(key) # rubocop: disable GitlabSecurity/PublicSend
+        return unless record
+
+        serialized_record = serialize_record(key, record, options)
+        return unless serialized_record
+
+        # `#as_json` always returns keys as `strings`
+        [key.to_s, serialized_record]
+      end
+
+      def serialize_record(key, record, options)
+        unless record.respond_to?(:as_json)
+          raise "Invalid type of #{key} is #{record.class}"
+        end
+
+        # no has-many relation
+        unless record.is_a?(ActiveRecord::Relation)
+          return record.as_json(options)
+        end
+
+        # has-many relation
+        data = []
+
+        record.in_batches(of: @batch_size) do |batch| # rubocop:disable Cop/InBatches
+          batch = batch.preload(preloads[key]) if preloads&.key?(key)
+          data += batch.as_json(options)
+        end
+
+        data
+      end
+
+      def includes
+        tree[:include]
+      end
+
+      def preloads
+        tree[:preload]
+      end
+    end
+  end
+end
diff --git a/lib/gitlab/import_export/import_export.yml b/lib/gitlab/import_export/import_export.yml
index 06c94beead8..511b702553e 100644
--- a/lib/gitlab/import_export/import_export.yml
+++ b/lib/gitlab/import_export/import_export.yml
@@ -231,6 +231,16 @@ methods:
   ci_pipelines:
     - :notes
 
+preloads:
+  statuses:
+    # TODO: We cannot preload tags, as they are not part of `GenericCommitStatus`
+    # tags: # needed by tag_list
+    project: # deprecated: needed by coverage_regex of Ci::Build
+  merge_requests:
+    source_project: # needed by source_branch_sha and diff_head_sha
+    target_project: # needed by target_branch_sha
+    assignees: # needed by assigne_id that is implemented by DeprecatedAssignee
+
 # EE specific relationships and settings to include. All of this will be merged
 # into the previous structures if EE is used.
 ee:
diff --git a/lib/gitlab/import_export/project_tree_saver.rb b/lib/gitlab/import_export/project_tree_saver.rb
index f1b3db6b208..f75f69b2c75 100644
--- a/lib/gitlab/import_export/project_tree_saver.rb
+++ b/lib/gitlab/import_export/project_tree_saver.rb
@@ -41,7 +41,13 @@ module Gitlab
       end
 
       def serialize_project_tree
-        @project.as_json(reader.project_tree)
+        if Feature.enabled?(:export_fast_serialize, default_enabled: true)
+          Gitlab::ImportExport::FastHashSerializer
+            .new(@project, reader.project_tree)
+            .execute
+        else
+          @project.as_json(reader.project_tree)
+        end
       end
 
       def reader
diff --git a/spec/lib/gitlab/import_export/attributes_finder_spec.rb b/spec/lib/gitlab/import_export/attributes_finder_spec.rb
index 208b60844e3..3cbc1375d6e 100644
--- a/spec/lib/gitlab/import_export/attributes_finder_spec.rb
+++ b/spec/lib/gitlab/import_export/attributes_finder_spec.rb
@@ -20,20 +20,41 @@ describe Gitlab::ImportExport::AttributesFinder do
             except: [:iid],
             include: [
               { merge_request_diff: {
-                include: []
+                include: [],
+                preload: { source_project: nil }
               } },
               { merge_request_test: { include: [] } }
             ],
-            only: [:id]
+            only: [:id],
+            preload: {
+              merge_request_diff: { source_project: nil },
+              merge_request_test: nil
+            }
             } },
           { commit_statuses: {
-              include: [{ commit: { include: [] } }]
+              include: [{ commit: { include: [] } }],
+              preload: { commit: nil }
             } },
           { project_members: {
               include: [{ user: { include: [],
-                                  only: [:email] } }]
+                                  only: [:email] } }],
+              preload: { user: nil }
           } }
-        ]
+        ],
+        preload: {
+          commit_statuses: {
+            commit: nil
+          },
+          issues: nil,
+          labels: nil,
+          merge_requests: {
+            merge_request_diff: { source_project: nil },
+            merge_request_test: nil
+          },
+          project_members: {
+            user: nil
+          }
+        }
       }
     end
 
@@ -50,7 +71,8 @@ describe Gitlab::ImportExport::AttributesFinder do
         setup_yaml(tree: { project: [:issues] })
 
         is_expected.to match(
-          include: [{ issues: { include: [] } }]
+          include: [{ issues: { include: [] } }],
+          preload: { issues: nil }
         )
       end
 
@@ -58,7 +80,8 @@ describe Gitlab::ImportExport::AttributesFinder do
         setup_yaml(tree: { project: [:project_feature] })
 
         is_expected.to match(
-          include: [{ project_feature: { include: [] } }]
+          include: [{ project_feature: { include: [] } }],
+          preload: { project_feature: nil }
         )
       end
 
@@ -67,7 +90,8 @@ describe Gitlab::ImportExport::AttributesFinder do
 
         is_expected.to match(
           include: [{ issues: { include: [] } },
-                    { snippets: { include: [] } }]
+                    { snippets: { include: [] } }],
+          preload: { issues: nil, snippets: nil }
         )
       end
 
@@ -75,7 +99,9 @@ describe Gitlab::ImportExport::AttributesFinder do
         setup_yaml(tree: { project: [issues: [:notes]] })
 
         is_expected.to match(
-          include: [{ issues: { include: [{ notes: { include: [] } }] } }]
+          include: [{ issues: { include: [{ notes: { include: [] } }],
+                                preload: { notes: nil } } }],
+          preload: { issues: { notes: nil } }
         )
       end
 
@@ -85,7 +111,9 @@ describe Gitlab::ImportExport::AttributesFinder do
         is_expected.to match(
           include: [{ merge_requests:
                       { include: [{ notes: { include: [] } },
-                                  { merge_request_diff: { include: [] } }] } }]
+                                  { merge_request_diff: { include: [] } }],
+                        preload: { merge_request_diff: nil, notes: nil } } }],
+          preload: { merge_requests: { merge_request_diff: nil, notes: nil } }
         )
       end
 
@@ -94,8 +122,11 @@ describe Gitlab::ImportExport::AttributesFinder do
 
         is_expected.to match(
           include: [{ merge_requests: {
-                      include: [{ notes: { include: [{ author: { include: [] } }] } }]
-                    } }]
+                      include: [{ notes: { include: [{ author: { include: [] } }],
+                                          preload: { author: nil } } }],
+                      preload: { notes: { author: nil } }
+                    } }],
+          preload: { merge_requests: { notes: { author: nil } } }
         )
       end
 
@@ -105,7 +136,8 @@ describe Gitlab::ImportExport::AttributesFinder do
 
         is_expected.to match(
           include: [{ issues: { include: [],
-                                only: [:name, :description] } }]
+                                only: [:name, :description] } }],
+          preload: { issues: nil }
         )
       end
 
@@ -115,7 +147,8 @@ describe Gitlab::ImportExport::AttributesFinder do
 
         is_expected.to match(
           include: [{ issues: { except: [:name],
-                                include: [] } }]
+                                include: [] } }],
+          preload: { issues: nil }
         )
       end
 
@@ -127,7 +160,8 @@ describe Gitlab::ImportExport::AttributesFinder do
         is_expected.to match(
           include: [{ issues: { except: [:name],
                                 include: [],
-                                only: [:description] } }]
+                                only: [:description] } }],
+          preload: { issues: nil }
         )
       end
 
@@ -137,7 +171,8 @@ describe Gitlab::ImportExport::AttributesFinder do
 
         is_expected.to match(
           include: [{ issues: { include: [],
-                                methods: [:name] } }]
+                                methods: [:name] } }],
+          preload: { issues: nil }
         )
       end
 
diff --git a/spec/lib/gitlab/import_export/config_spec.rb b/spec/lib/gitlab/import_export/config_spec.rb
index e53db37def4..f09a29b84db 100644
--- a/spec/lib/gitlab/import_export/config_spec.rb
+++ b/spec/lib/gitlab/import_export/config_spec.rb
@@ -25,7 +25,7 @@ describe Gitlab::ImportExport::Config do
           expect { subject }.not_to raise_error
           expect(subject).to be_a(Hash)
           expect(subject.keys).to contain_exactly(
-            :tree, :excluded_attributes, :included_attributes, :methods)
+            :tree, :excluded_attributes, :included_attributes, :methods, :preloads)
         end
       end
     end
@@ -55,6 +55,10 @@ describe Gitlab::ImportExport::Config do
             events:
               - :action
 
+          preloads:
+            statuses:
+              project:
+
           ee:
             tree:
               project:
@@ -71,6 +75,9 @@ describe Gitlab::ImportExport::Config do
                 - :type_ee
               events_ee:
                 - :action_ee
+            preloads:
+              statuses:
+                bridge_ee:
         EOF
       end
 
@@ -111,6 +118,11 @@ describe Gitlab::ImportExport::Config do
               methods: {
                 labels: [:type],
                 events: [:action]
+              },
+              preloads: {
+                statuses: {
+                  project: nil
+                }
               }
             }
           )
@@ -150,6 +162,12 @@ describe Gitlab::ImportExport::Config do
                 labels: [:type, :type_ee],
                 events: [:action],
                 events_ee: [:action_ee]
+              },
+              preloads: {
+                statuses: {
+                  project: nil,
+                  bridge_ee: nil
+                }
               }
             }
           )
diff --git a/spec/lib/gitlab/import_export/fast_hash_serializer_spec.rb b/spec/lib/gitlab/import_export/fast_hash_serializer_spec.rb
new file mode 100644
index 00000000000..d23b27c9d8e
--- /dev/null
+++ b/spec/lib/gitlab/import_export/fast_hash_serializer_spec.rb
@@ -0,0 +1,272 @@
+require 'spec_helper'
+
+describe Gitlab::ImportExport::FastHashSerializer do
+  subject { described_class.new(project, tree).execute }
+
+  let!(:project) { setup_project }
+  let(:user) { create(:user) }
+  let(:shared) { project.import_export_shared }
+  let(:reader) { Gitlab::ImportExport::Reader.new(shared: shared) }
+  let(:tree) { reader.project_tree }
+
+  before do
+    project.add_maintainer(user)
+    allow_any_instance_of(MergeRequest).to receive(:source_branch_sha).and_return('ABCD')
+    allow_any_instance_of(MergeRequest).to receive(:target_branch_sha).and_return('DCBA')
+  end
+
+  it 'saves the correct hash' do
+    is_expected.to include({ 'description' => 'description', 'visibility_level' => 20 })
+  end
+
+  it 'has approvals_before_merge set' do
+    expect(subject['approvals_before_merge']).to eq(1)
+  end
+
+  it 'has milestones' do
+    expect(subject['milestones']).not_to be_empty
+  end
+
+  it 'has merge requests' do
+    expect(subject['merge_requests']).not_to be_empty
+  end
+
+  it 'has merge request\'s milestones' do
+    expect(subject['merge_requests'].first['milestone']).not_to be_empty
+  end
+
+  it 'has merge request\'s source branch SHA' do
+    expect(subject['merge_requests'].first['source_branch_sha']).to eq('ABCD')
+  end
+
+  it 'has merge request\'s target branch SHA' do
+    expect(subject['merge_requests'].first['target_branch_sha']).to eq('DCBA')
+  end
+
+  it 'has events' do
+    expect(subject['merge_requests'].first['milestone']['events']).not_to be_empty
+  end
+
+  it 'has snippets' do
+    expect(subject['snippets']).not_to be_empty
+  end
+
+  it 'has snippet notes' do
+    expect(subject['snippets'].first['notes']).not_to be_empty
+  end
+
+  it 'has releases' do
+    expect(subject['releases']).not_to be_empty
+  end
+
+  it 'has no author on releases' do
+    expect(subject['releases'].first['author']).to be_nil
+  end
+
+  it 'has the author ID on releases' do
+    expect(subject['releases'].first['author_id']).not_to be_nil
+  end
+
+  it 'has issues' do
+    expect(subject['issues']).not_to be_empty
+  end
+
+  it 'has issue comments' do
+    notes = subject['issues'].first['notes']
+
+    expect(notes).not_to be_empty
+    expect(notes.first['type']).to eq('DiscussionNote')
+  end
+
+  it 'has issue assignees' do
+    expect(subject['issues'].first['issue_assignees']).not_to be_empty
+  end
+
+  it 'has author on issue comments' do
+    expect(subject['issues'].first['notes'].first['author']).not_to be_empty
+  end
+
+  it 'has project members' do
+    expect(subject['project_members']).not_to be_empty
+  end
+
+  it 'has merge requests diffs' do
+    expect(subject['merge_requests'].first['merge_request_diff']).not_to be_empty
+  end
+
+  it 'has merge request diff files' do
+    expect(subject['merge_requests'].first['merge_request_diff']['merge_request_diff_files']).not_to be_empty
+  end
+
+  it 'has merge request diff commits' do
+    expect(subject['merge_requests'].first['merge_request_diff']['merge_request_diff_commits']).not_to be_empty
+  end
+
+  it 'has merge requests comments' do
+    expect(subject['merge_requests'].first['notes']).not_to be_empty
+  end
+
+  it 'has author on merge requests comments' do
+    expect(subject['merge_requests'].first['notes'].first['author']).not_to be_empty
+  end
+
+  it 'has pipeline stages' do
+    expect(subject.dig('ci_pipelines', 0, 'stages')).not_to be_empty
+  end
+
+  it 'has pipeline statuses' do
+    expect(subject.dig('ci_pipelines', 0, 'stages', 0, 'statuses')).not_to be_empty
+  end
+
+  it 'has pipeline builds' do
+    builds_count = subject
+      .dig('ci_pipelines', 0, 'stages', 0, 'statuses')
+      .count { |hash| hash['type'] == 'Ci::Build' }
+
+    expect(builds_count).to eq(1)
+  end
+
+  it 'has no when YML attributes but only the DB column' do
+    allow_any_instance_of(Ci::Pipeline)
+      .to receive(:ci_yaml_file)
+      .and_return(File.read(Rails.root.join('spec/support/gitlab_stubs/gitlab_ci.yml')))
+
+    expect_any_instance_of(Gitlab::Ci::YamlProcessor).not_to receive(:build_attributes)
+
+    subject
+  end
+
+  it 'has pipeline commits' do
+    expect(subject['ci_pipelines']).not_to be_empty
+  end
+
+  it 'has ci pipeline notes' do
+    expect(subject['ci_pipelines'].first['notes']).not_to be_empty
+  end
+
+  it 'has labels with no associations' do
+    expect(subject['labels']).not_to be_empty
+  end
+
+  it 'has labels associated to records' do
+    expect(subject['issues'].first['label_links'].first['label']).not_to be_empty
+  end
+
+  it 'has project and group labels' do
+    label_types = subject['issues'].first['label_links'].map { |link| link['label']['type'] }
+
+    expect(label_types).to match_array(%w(ProjectLabel GroupLabel))
+  end
+
+  it 'has priorities associated to labels' do
+    priorities = subject['issues'].first['label_links'].flat_map { |link| link['label']['priorities'] }
+
+    expect(priorities).not_to be_empty
+  end
+
+  it 'has issue resource label events' do
+    expect(subject['issues'].first['resource_label_events']).not_to be_empty
+  end
+
+  it 'has merge request resource label events' do
+    expect(subject['merge_requests'].first['resource_label_events']).not_to be_empty
+  end
+
+  it 'saves the correct service type' do
+    expect(subject['services'].first['type']).to eq('CustomIssueTrackerService')
+  end
+
+  it 'saves the properties for a service' do
+    expect(subject['services'].first['properties']).to eq('one' => 'value')
+  end
+
+  it 'has project feature' do
+    project_feature = subject['project_feature']
+    expect(project_feature).not_to be_empty
+    expect(project_feature["issues_access_level"]).to eq(ProjectFeature::DISABLED)
+    expect(project_feature["wiki_access_level"]).to eq(ProjectFeature::ENABLED)
+    expect(project_feature["builds_access_level"]).to eq(ProjectFeature::PRIVATE)
+  end
+
+  it 'has custom attributes' do
+    expect(subject['custom_attributes'].count).to eq(2)
+  end
+
+  it 'has badges' do
+    expect(subject['project_badges'].count).to eq(2)
+  end
+
+  it 'does not complain about non UTF-8 characters in MR diff files' do
+    ActiveRecord::Base.connection.execute("UPDATE merge_request_diff_files SET diff = '---\n- :diff: !binary |-\n    LS0tIC9kZXYvbnVsbAorKysgYi9pbWFnZXMvbnVjb3IucGRmCkBAIC0wLDAg\n    KzEsMTY3OSBAQAorJVBERi0xLjUNJeLjz9MNCisxIDAgb2JqDTw8L01ldGFk\n    YXR'")
+
+    expect(subject['merge_requests'].first['merge_request_diff']).not_to be_empty
+  end
+
+  context 'project attributes' do
+    it 'does not contain the runners token' do
+      expect(subject).not_to include("runners_token" => 'token')
+    end
+  end
+
+  it 'has a board and a list' do
+    expect(subject['boards'].first['lists']).not_to be_empty
+  end
+
+  def setup_project
+    issue = create(:issue, assignees: [user])
+    snippet = create(:project_snippet)
+    release = create(:release)
+    group = create(:group)
+
+    project = create(:project,
+                     :public,
+                     :repository,
+                     :issues_disabled,
+                     :wiki_enabled,
+                     :builds_private,
+                     description: 'description',
+                     issues: [issue],
+                     snippets: [snippet],
+                     releases: [release],
+                     group: group,
+                     approvals_before_merge: 1
+                    )
+    project_label = create(:label, project: project)
+    group_label = create(:group_label, group: group)
+    create(:label_link, label: project_label, target: issue)
+    create(:label_link, label: group_label, target: issue)
+    create(:label_priority, label: group_label, priority: 1)
+    milestone = create(:milestone, project: project)
+    merge_request = create(:merge_request, source_project: project, milestone: milestone)
+
+    ci_build = create(:ci_build, project: project, when: nil)
+    ci_build.pipeline.update(project: project)
+    create(:commit_status, project: project, pipeline: ci_build.pipeline)
+
+    create(:milestone, project: project)
+    create(:discussion_note, noteable: issue, project: project)
+    create(:note, noteable: merge_request, project: project)
+    create(:note, noteable: snippet, project: project)
+    create(:note_on_commit,
+           author: user,
+           project: project,
+           commit_id: ci_build.pipeline.sha)
+
+    create(:resource_label_event, label: project_label, issue: issue)
+    create(:resource_label_event, label: group_label, merge_request: merge_request)
+
+    create(:event, :created, target: milestone, project: project, author: user)
+    create(:service, project: project, type: 'CustomIssueTrackerService', category: 'issue_tracker', properties: { one: 'value' })
+
+    create(:project_custom_attribute, project: project)
+    create(:project_custom_attribute, project: project)
+
+    create(:project_badge, project: project)
+    create(:project_badge, project: project)
+
+    board = create(:board, project: project, name: 'TestBoard')
+    create(:list, board: board, position: 0, label: project_label)
+
+    project
+  end
+end
diff --git a/spec/lib/gitlab/import_export/project_tree_saver_spec.rb b/spec/lib/gitlab/import_export/project_tree_saver_spec.rb
index fefbed93316..ff46e062a5d 100644
--- a/spec/lib/gitlab/import_export/project_tree_saver_spec.rb
+++ b/spec/lib/gitlab/import_export/project_tree_saver_spec.rb
@@ -23,12 +23,65 @@ describe Gitlab::ImportExport::ProjectTreeSaver do
       expect(project_tree_saver.save).to be true
     end
 
+    context ':export_fast_serialize feature flag checks' do
+      before do
+        expect(Gitlab::ImportExport::Reader).to receive(:new).with(shared: shared).and_return(reader)
+        expect(reader).to receive(:project_tree).and_return(project_tree)
+      end
+
+      let(:serializer) { instance_double('Gitlab::ImportExport::FastHashSerializer') }
+      let(:reader) { instance_double('Gitlab::ImportExport::Reader') }
+      let(:project_tree) do
+        {
+          include: [{ issues: { include: [] } }],
+          preload: { issues: nil }
+        }
+      end
+
+      context 'when :export_fast_serialize feature is enabled' do
+        before do
+          stub_feature_flags(export_fast_serialize: true)
+        end
+
+        it 'uses FastHashSerializer' do
+          expect(Gitlab::ImportExport::FastHashSerializer)
+            .to receive(:new)
+            .with(project, project_tree)
+            .and_return(serializer)
+
+          expect(serializer).to receive(:execute)
+
+          project_tree_saver.save
+        end
+      end
+
+      context 'when :export_fast_serialize feature is disabled' do
+        before do
+          stub_feature_flags(export_fast_serialize: false)
+        end
+
+        it 'is serialized via built-in `as_json`' do
+          expect(project).to receive(:as_json).with(project_tree)
+
+          project_tree_saver.save
+        end
+      end
+    end
+
+    # It is mostly duplicated in
+    # `spec/lib/gitlab/import_export/fast_hash_serializer_spec.rb`
+    # except:
+    # context 'with description override' do
+    # context 'group members' do
+    # ^ These are specific for the ProjectTreeSaver
     context 'JSON' do
       let(:saved_project_json) do
         project_tree_saver.save
         project_json(project_tree_saver.full_path)
       end
 
+      # It is not duplicated in
+      # `spec/lib/gitlab/import_export/fast_hash_serializer_spec.rb`
       context 'with description override' do
         let(:params) { { description: 'Foo Bar' } }
         let(:project_tree_saver) { described_class.new(project: project, current_user: user, shared: shared, params: params) }
diff --git a/spec/support/import_export/import_export.yml b/spec/support/import_export/import_export.yml
index ed2a3243f0d..116bc8d0b9c 100644
--- a/spec/support/import_export/import_export.yml
+++ b/spec/support/import_export/import_export.yml
@@ -13,6 +13,10 @@ tree:
   group_members:
     - :user
 
+preloads:
+  merge_request_diff:
+    source_project:
+
 included_attributes:
   merge_requests:
     - :id