Significantly reduce N+1 queries in /api/v4/todos endpoint

By preloading associations and batching issuable metadata lookups,
we can significantly cut the number of SQL queries needed to load
the Todos API endpoint.

On GitLab.com, my own tests showed my user's SQL queries went
from 365 to under 60 SQL queries.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/40378

1 files changed, 47 insertions, 5 deletions

diff --git a/spec/requests/api/todos_spec.rb b/spec/requests/api/todos_spec.rb
index f121a1d3b78..9f0d5ad5d12 100644
--- a/spec/requests/api/todos_spec.rb
+++ b/spec/requests/api/todos_spec.rb
@@ -8,10 +8,14 @@ describe API::Todos do
   let(:author_2) { create(:user) }
   let(:john_doe) { create(:user, username: 'john_doe') }
   let(:merge_request) { create(:merge_request, source_project: project_1) }
+  let!(:merge_request_todo) { create(:todo, project: project_1, author: author_2, user: john_doe, target: merge_request) }
   let!(:pending_1) { create(:todo, :mentioned, project: project_1, author: author_1, user: john_doe) }
   let!(:pending_2) { create(:todo, project: project_2, author: author_2, user: john_doe) }
   let!(:pending_3) { create(:on_commit_todo, project: project_1, author: author_2, user: john_doe) }
   let!(:done) { create(:todo, :done, project: project_1, author: author_1, user: john_doe) }
+  let!(:award_emoji_1) { create(:award_emoji, awardable: merge_request, user: author_1, name: 'thumbsup') }
+  let!(:award_emoji_2) { create(:award_emoji, awardable: pending_1.target, user: author_1, name: 'thumbsup') }
+  let!(:award_emoji_3) { create(:award_emoji, awardable: pending_2.target, user: author_2, name: 'thumbsdown') }
 
   before do
     project_1.add_developer(john_doe)
@@ -34,7 +38,7 @@ describe API::Todos do
         expect(response.status).to eq(200)
         expect(response).to include_pagination_headers
         expect(json_response).to be_an Array
-        expect(json_response.length).to eq(3)
+        expect(json_response.length).to eq(4)
         expect(json_response[0]['id']).to eq(pending_3.id)
         expect(json_response[0]['project']).to be_a Hash
         expect(json_response[0]['author']).to be_a Hash
@@ -45,6 +49,23 @@ describe API::Todos do
         expect(json_response[0]['state']).to eq('pending')
         expect(json_response[0]['action_name']).to eq('assigned')
         expect(json_response[0]['created_at']).to be_present
+        expect(json_response[0]['target_type']).to eq('Commit')
+
+        expect(json_response[1]['target_type']).to eq('Issue')
+        expect(json_response[1]['target']['upvotes']).to eq(0)
+        expect(json_response[1]['target']['downvotes']).to eq(1)
+        expect(json_response[1]['target']['merge_requests_count']).to eq(0)
+
+        expect(json_response[2]['target_type']).to eq('Issue')
+        expect(json_response[2]['target']['upvotes']).to eq(1)
+        expect(json_response[2]['target']['downvotes']).to eq(0)
+        expect(json_response[2]['target']['merge_requests_count']).to eq(0)
+
+        expect(json_response[3]['target_type']).to eq('MergeRequest')
+        # Only issues get a merge request count at the moment
+        expect(json_response[3]['target']['merge_requests_count']).to be_nil
+        expect(json_response[3]['target']['upvotes']).to eq(1)
+        expect(json_response[3]['target']['downvotes']).to eq(0)
       end
 
       context 'and using the author filter' do
@@ -54,7 +75,7 @@ describe API::Todos do
           expect(response.status).to eq(200)
           expect(response).to include_pagination_headers
           expect(json_response).to be_an Array
-          expect(json_response.length).to eq(2)
+          expect(json_response.length).to eq(3)
         end
       end
 
@@ -67,7 +88,7 @@ describe API::Todos do
           expect(response.status).to eq(200)
           expect(response).to include_pagination_headers
           expect(json_response).to be_an Array
-          expect(json_response.length).to eq(1)
+          expect(json_response.length).to eq(2)
         end
       end
 
@@ -100,7 +121,7 @@ describe API::Todos do
           expect(response.status).to eq(200)
           expect(response).to include_pagination_headers
           expect(json_response).to be_an Array
-          expect(json_response.length).to eq(2)
+          expect(json_response.length).to eq(3)
         end
       end
 
@@ -115,6 +136,27 @@ describe API::Todos do
         end
       end
     end
+
+    it 'avoids N+1 queries', :request_store do
+      create(:todo, project: project_1, author: author_2, user: john_doe, target: merge_request)
+
+      get api('/todos', john_doe)
+
+      control = ActiveRecord::QueryRecorder.new { get api('/todos', john_doe) }
+
+      merge_request_2 = create(:merge_request, source_project: project_2)
+      create(:todo, project: project_2, author: author_2, user: john_doe, target: merge_request_2)
+
+      project_3 = create(:project, :repository)
+      project_3.add_developer(john_doe)
+      merge_request_3 = create(:merge_request, source_project: project_3)
+      create(:todo, project: project_3, author: author_2, user: john_doe, target: merge_request_3)
+      create(:todo, :mentioned, project: project_1, author: author_1, user: john_doe)
+      create(:on_commit_todo, project: project_3, author: author_1, user: john_doe)
+
+      expect { get api('/todos', john_doe) }.not_to exceed_query_limit(control)
+      expect(response.status).to eq(200)
+    end
   end
 
   describe 'POST /todos/:id/mark_as_done' do
@@ -230,7 +272,7 @@ describe API::Todos do
 
     context 'for a merge request' do
       it_behaves_like 'an issuable', 'merge_requests' do
-        let(:issuable) { merge_request }
+        let(:issuable) { create(:merge_request, :simple, source_project: project_1) }
       end
     end
   end