Improve performance and memory usage of project exportkamil-improve-import-export

ActiveModel::Serialization 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. We use `tree:` and `preload:` to automatically generate
   a list of all preloads that could be used to serialize
   objects in bulk.

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.

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