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

app/models/issue.rb

@@ -64,6 +64,7 @@ class Issue < ActiveRecord::Base
   scope :order_closest_future_date, -> { reorder('CASE WHEN issues.due_date >= CURRENT_DATE THEN 0 ELSE 1 END ASC, ABS(CURRENT_DATE - issues.due_date) ASC') }
 
   scope :preload_associations, -> { preload(:labels, project: :namespace) }
+  scope :with_api_entity_associations, -> { preload(:timelogs, :assignees, :author, :notes, :labels, project: [:route, { namespace: :route }] ) }
 
   scope :public_only, -> { where(confidential: false) }
   scope :confidential_only, -> { where(confidential: true) }

app/models/merge_request.rb

@@ -184,6 +184,13 @@ class MergeRequest < ActiveRecord::Base
   scope :assigned, -> { where("assignee_id IS NOT NULL") }
   scope :unassigned, -> { where("assignee_id IS NULL") }
   scope :assigned_to, ->(u) { where(assignee_id: u.id)}
+  scope :with_api_entity_associations, -> {
+    preload(:author, :assignee, :notes, :labels, :milestone, :timelogs,
+            latest_merge_request_diff: [:merge_request_diff_commits],
+            metrics: [:latest_closed_by, :merged_by],
+            target_project: [:route, { namespace: :route }],
+            source_project: [:route, { namespace: :route }])
+  }
 
   participant :assignee
 

app/models/todo.rb

@@ -31,7 +31,13 @@ class Todo < ActiveRecord::Base
   belongs_to :note
   belongs_to :project
   belongs_to :group
-  belongs_to :target, polymorphic: true, touch: true # rubocop:disable Cop/PolymorphicAssociations
+  belongs_to :target, -> {
+    if self.klass.respond_to?(:with_api_entity_associations)
+      self.with_api_entity_associations
+    else
+      self
+    end
+  }, polymorphic: true, touch: true # rubocop:disable Cop/PolymorphicAssociations
   belongs_to :user
 
   delegate :name, :email, to: :author, prefix: true, allow_nil: true
@@ -52,6 +58,7 @@ class Todo < ActiveRecord::Base
   scope :for_type, -> (type) { where(target_type: type) }
   scope :for_target, -> (id) { where(target_id: id) }
   scope :for_commit, -> (id) { where(commit_id: id) }
+  scope :with_api_entity_associations, -> { preload(:target, :author, :note, group: :route, project: [:route, { namespace: :route }]) }
 
   state_machine :state, initial: :pending do
     event :done do

changelogs/unreleased/sh-optimize-todos-api.yml

+---
+title: Significantly reduce N+1 queries in /api/v4/todos endpoint
+merge_request: 25711
+author:
+type: performance

lib/api/entities.rb

@@ -883,7 +883,8 @@ module API
       expose :target_type
 
       expose :target do |todo, options|
-        todo_target_class(todo.target_type).represent(todo.target, options)
+        todo_options = options.fetch(todo.target_type, {})
+        todo_target_class(todo.target_type).represent(todo.target, todo_options)
       end
 
       expose :target_url do |todo, options|

lib/api/issues.rb

@@ -28,7 +28,7 @@ module API
         args[:scope] = args[:scope].underscore if args[:scope]
 
         issues = IssuesFinder.new(current_user, args).execute
-          .preload(:assignees, :labels, :notes, :timelogs, :project, :author, :closed_by)
+                   .with_api_entity_associations
         issues.reorder(order_options_with_tie_breaker)
       end
       # rubocop: enable CodeReuse/ActiveRecord

lib/api/merge_requests.rb

@@ -45,7 +45,7 @@ module API
         return merge_requests if args[:view] == 'simple'
 
         merge_requests
-          .preload(:notes, :author, :assignee, :milestone, :latest_merge_request_diff, :labels, :timelogs, metrics: [:latest_closed_by, :merged_by])
+          .with_api_entity_associations
       end
       # rubocop: enable CodeReuse/ActiveRecord
 

lib/api/todos.rb

@@ -6,6 +6,8 @@ module API
 
     before { authenticate! }
 
+    helpers ::Gitlab::IssuableMetadata
+
     ISSUABLE_TYPES = {
       'merge_requests' => ->(iid) { find_merge_request_with_access(iid) },
       'issues' => ->(iid) { find_project_issue(iid) }
@@ -42,6 +44,30 @@ module API
         def find_todos
           TodosFinder.new(current_user, params).execute
         end
+
+        def issuable_and_awardable?(type)
+          obj_type = Object.const_get(type)
+
+          (obj_type < Issuable) && (obj_type < Awardable)
+        rescue NameError
+          false
+        end
+
+        def batch_load_issuable_metadata(todos, options)
+          # This should be paginated and will cause Rails to SELECT for all the Todos
+          todos_by_type = todos.group_by(&:target_type)
+
+          todos_by_type.keys.each do |type|
+            next unless issuable_and_awardable?(type)
+
+            collection = todos_by_type[type]
+
+            next unless collection
+
+            targets = collection.map(&:target)
+            options[type] = { issuable_metadata: issuable_meta_data(targets, type) }
+          end
+        end
       end
 
       desc 'Get a todo list' do
@@ -51,7 +77,11 @@ module API
         use :pagination
       end
       get do
-        present paginate(find_todos), with: Entities::Todo, current_user: current_user
+        todos = paginate(find_todos.with_api_entity_associations)
+        options = { with: Entities::Todo, current_user: current_user }
+        batch_load_issuable_metadata(todos, options)
+
+        present todos, options
       end
 
       desc 'Mark a todo as done' do

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