Denormalize commits count for merge request diffs

For each MR diff an extra 'SELECT COUNT()' is executed
to get number of commits for the diff. Overall time to get counts for
all MR diffs may be quite expensive. To speed up loading of MR info,
information about number of commits is stored in a MR diff's extra column.

Closes #38068

3 files changed, 61 insertions, 0 deletions

diff --git a/spec/lib/gitlab/background_migration/add_merge_request_diff_commits_count_spec.rb b/spec/lib/gitlab/background_migration/add_merge_request_diff_commits_count_spec.rb
new file mode 100644
index 00000000000..21a791f5695
--- /dev/null
+++ b/spec/lib/gitlab/background_migration/add_merge_request_diff_commits_count_spec.rb
@@ -0,0 +1,50 @@
+require 'spec_helper'
+
+describe Gitlab::BackgroundMigration::AddMergeRequestDiffCommitsCount, :migration, schema: 20180105212544 do
+  let(:projects_table) { table(:projects) }
+  let(:merge_requests_table) { table(:merge_requests) }
+  let(:merge_request_diffs_table) { table(:merge_request_diffs) }
+  let(:merge_request_diff_commits_table) { table(:merge_request_diff_commits) }
+
+  let(:project) { projects_table.create!(name: 'gitlab', path: 'gitlab-org/gitlab-ce') }
+  let(:merge_request) do
+    merge_requests_table.create!(target_project_id: project.id,
+                                 target_branch: 'master',
+                                 source_project_id: project.id,
+                                 source_branch: 'mr name',
+                                 title: 'mr name')
+  end
+
+  def create_diff!(name, commits: 0)
+    mr_diff = merge_request_diffs_table.create!(
+      merge_request_id: merge_request.id)
+
+    commits.times do |i|
+      merge_request_diff_commits_table.create!(
+        merge_request_diff_id: mr_diff.id,
+        relative_order: i, sha: i)
+    end
+
+    mr_diff
+  end
+
+  describe '#perform' do
+    it 'migrates diffs that have no commits' do
+      diff = create_diff!('with_multiple_commits', commits: 0)
+
+      subject.perform(diff.id, diff.id)
+
+      expect(diff.reload.commits_count).to eq(0)
+    end
+
+    it 'migrates multiple diffs to the correct values' do
+      diffs = Array.new(3).map.with_index { |_, i| create_diff!(i, commits: 3) }
+
+      subject.perform(diffs.first.id, diffs.last.id)
+
+      diffs.each do |diff|
+        expect(diff.reload.commits_count).to eq(3)
+      end
+    end
+  end
+end
diff --git a/spec/lib/gitlab/background_migration/deserialize_merge_request_diffs_and_commits_spec.rb b/spec/lib/gitlab/background_migration/deserialize_merge_request_diffs_and_commits_spec.rb
index 84d9e635810..98730602863 100644
--- a/spec/lib/gitlab/background_migration/deserialize_merge_request_diffs_and_commits_spec.rb
+++ b/spec/lib/gitlab/background_migration/deserialize_merge_request_diffs_and_commits_spec.rb
@@ -10,6 +10,11 @@ describe Gitlab::BackgroundMigration::DeserializeMergeRequestDiffsAndCommits, :t
     let(:merge_request_diff) { MergeRequest.find(merge_request.id).create_merge_request_diff }
     let(:updated_merge_request_diff) { MergeRequestDiff.find(merge_request_diff.id) }
 
+    before do
+      allow_any_instance_of(MergeRequestDiff)
+        .to receive(:commits_count=).and_return(nil)
+    end
+
     def diffs_to_hashes(diffs)
       diffs.as_json(only: Gitlab::Git::Diff::SERIALIZE_KEYS).map(&:with_indifferent_access)
     end
diff --git a/spec/lib/gitlab/background_migration/populate_merge_request_metrics_with_events_data_spec.rb b/spec/lib/gitlab/background_migration/populate_merge_request_metrics_with_events_data_spec.rb
index dfe3b31f1c0..e99257e3481 100644
--- a/spec/lib/gitlab/background_migration/populate_merge_request_metrics_with_events_data_spec.rb
+++ b/spec/lib/gitlab/background_migration/populate_merge_request_metrics_with_events_data_spec.rb
@@ -1,6 +1,12 @@
 require 'rails_helper'
 
 describe Gitlab::BackgroundMigration::PopulateMergeRequestMetricsWithEventsData, :migration, schema: 20171128214150 do
+  # commits_count attribute is added in a next migration
+  before do
+    allow_any_instance_of(MergeRequestDiff)
+      .to receive(:commits_count=).and_return(nil)
+  end
+
   describe '#perform' do
     let(:mr_with_event) { create(:merge_request) }
     let!(:merged_event) { create(:event, :merged, target: mr_with_event) }