1 files changed, 60 insertions, 0 deletions
diff --git a/spec/controllers/concerns/renders_commits_spec.rb b/spec/controllers/concerns/renders_commits_spec.rb
new file mode 100644
index 00000000000..79350847383
--- /dev/null
+++ b/spec/controllers/concerns/renders_commits_spec.rb
@@ -0,0 +1,60 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe RendersCommits do
+  let_it_be(:project) { create(:project, :public, :repository) }
+  let_it_be(:merge_request) { create(:merge_request, source_project: project) }
+  let_it_be(:user) { create(:user) }
+
+  controller(ApplicationController) do
+    # `described_class` is not available in this context
+    include RendersCommits # rubocop:disable RSpec/DescribedClass
+
+    def index
+      @merge_request = MergeRequest.find(params[:id])
+      @commits = set_commits_for_rendering(
+        @merge_request.recent_commits.with_latest_pipeline(@merge_request.source_branch),
+          commits_count: @merge_request.commits_count
+      )
+
+      render json: { html: view_to_html_string('projects/merge_requests/_commits') }
+    end
+  end
+
+  before do
+    sign_in(user)
+  end
+
+  def go
+    get :index, params: { id: merge_request.id }
+  end
+
+  it 'sets instance variables for counts' do
+    stub_const("MergeRequestDiff::COMMITS_SAFE_SIZE", 10)
+
+    go
+
+    expect(assigns[:total_commit_count]).to eq(29)
+    expect(assigns[:hidden_commit_count]).to eq(19)
+    expect(assigns[:commits].size).to eq(10)
+  end
+
+  context 'rendering commits' do
+    render_views
+
+    it 'avoids N + 1' do
+      stub_const("MergeRequestDiff::COMMITS_SAFE_SIZE", 5)
+
+      control_count = ActiveRecord::QueryRecorder.new(skip_cached: false) do
+        go
+      end.count
+
+      stub_const("MergeRequestDiff::COMMITS_SAFE_SIZE", 15)
+
+      expect do
+        go
+      end.not_to exceed_all_query_limit(control_count)
+    end
+  end
+end