Avoid N+1 queries when checking for pending todos

Before a new todo is created we check if the user already
has a pending todo for the item. If this operation is
executed for multiple users we will only use one query to
receive all pending todos

app/finders/pending_todos_finder.rb

@@ -11,17 +11,18 @@
 # change the various `by_*` methods in this finder, without having to touch
 # everything that uses it.
 class PendingTodosFinder
-  attr_reader :current_user, :params
+  attr_reader :users, :params
 
-  # current_user - The user to retrieve the todos for.
+  # users - The list of users to retrieve the todos for.
   # params - A Hash containing columns and values to use for filtering todos.
-  def initialize(current_user, params = {})
-    @current_user = current_user
+  def initialize(users, params = {})
+    @users = users
     @params = params
   end
 
   def execute
-    todos = current_user.todos.pending
+    todos = Todo.for_user(users)
+    todos = todos.pending
     todos = by_project(todos)
     todos = by_target_id(todos)
     todos = by_target_type(todos)

app/models/todo.rb

@@ -148,6 +148,10 @@ class Todo < ApplicationRecord
         .order(Gitlab::Database.nulls_last_order('highest_priority', 'ASC'))
         .order('todos.created_at')
     end
+
+    def pluck_user_id
+      pluck(:user_id)
+    end
   end
 
   def resource_parent

app/services/todo_service.rb

@@ -177,7 +177,7 @@ class TodoService
   def resolve_todos_for_target(target, current_user)
     attributes = attributes_for_target(target)
 
-    resolve_todos(pending_todos(current_user, attributes), current_user)
+    resolve_todos(pending_todos([current_user], attributes), current_user)
   end
 
   def resolve_todos(todos, current_user, resolution: :done, resolved_by_action: :system_done)
@@ -220,9 +220,14 @@ class TodoService
   private
 
   def create_todos(users, attributes)
-    Array(users).map do |user|
-      next if pending_todos(user, attributes).exists? && Feature.disabled?(:multiple_todos, user)
+    users = Array(users)
 
+    return if users.empty?
+
+    users_with_pending_todos = pending_todos(users, attributes).pluck_user_id
+    users.reject! { |user| users_with_pending_todos.include?(user.id) && Feature.disabled?(:multiple_todos, user) }
+
+    users.map do |user|
       issue_type = attributes.delete(:issue_type)
       track_todo_creation(user, issue_type)
 
@@ -353,8 +358,8 @@ class TodoService
     end
   end
 
-  def pending_todos(user, criteria = {})
-    PendingTodosFinder.new(user, criteria).execute
+  def pending_todos(users, criteria = {})
+    PendingTodosFinder.new(users, criteria).execute
   end
 
   def track_todo_creation(user, issue_type)

changelogs/unreleased/improve-todo-service-database-performance.yml

+---
+title: Reduce N+1 queries in creating todos after user mentions in a note
+merge_request: 57525
+author:
+type: performance

spec/finders/pending_todos_finder_spec.rb

@@ -4,13 +4,15 @@ require 'spec_helper'
 
 RSpec.describe PendingTodosFinder do
   let(:user) { create(:user) }
+  let(:user2) { create(:user) }
+  let(:users) { [user, user2] }
 
   describe '#execute' do
     it 'returns only pending todos' do
       create(:todo, :done, user: user)
 
       todo = create(:todo, :pending, user: user)
-      todos = described_class.new(user).execute
+      todos = described_class.new(users).execute
 
       expect(todos).to eq([todo])
     end
@@ -22,7 +24,7 @@ RSpec.describe PendingTodosFinder do
       create(:todo, :pending, user: user, project: project2)
 
       todo = create(:todo, :pending, user: user, project: project1)
-      todos = described_class.new(user, project_id: project1.id).execute
+      todos = described_class.new(users, project_id: project1.id).execute
 
       expect(todos).to eq([todo])
     end
@@ -34,7 +36,7 @@ RSpec.describe PendingTodosFinder do
 
       create(:todo, :pending, user: user, target: note)
 
-      todos = described_class.new(user, target_id: issue.id).execute
+      todos = described_class.new(users, target_id: issue.id).execute
 
       expect(todos).to eq([todo])
     end
@@ -46,7 +48,7 @@ RSpec.describe PendingTodosFinder do
 
       create(:todo, :pending, user: user, target: note)
 
-      todos = described_class.new(user, target_type: issue.class.name).execute
+      todos = described_class.new(users, target_type: issue.class.name).execute
 
       expect(todos).to eq([todo])
     end
@@ -55,7 +57,7 @@ RSpec.describe PendingTodosFinder do
       create(:todo, :pending, user: user, commit_id: '456')
 
       todo = create(:todo, :pending, user: user, commit_id: '123')
-      todos = described_class.new(user, commit_id: '123').execute
+      todos = described_class.new(users, commit_id: '123').execute
 
       expect(todos).to eq([todo])
     end

spec/models/todo_spec.rb

@@ -435,4 +435,12 @@ RSpec.describe Todo do
       end
     end
   end
+
+  describe '.pluck_user_id' do
+    subject { described_class.pluck_user_id }
+
+    let_it_be(:todo) { create(:todo) }
+
+    it { is_expected.to eq([todo.user_id]) }
+  end
 end

spec/services/todo_service_spec.rb

@@ -1007,7 +1007,8 @@ RSpec.describe TodoService do
   end
 
   describe '#update_note' do
-    let(:noteable) { create(:issue, project: project) }
+    let_it_be(:noteable) { create(:issue, project: project) }
+
     let(:note) { create(:note, project: project, note: mentions, noteable: noteable) }
     let(:addressed_note) { create(:note, project: project, note: "#{directly_addressed}", noteable: noteable) }
 
@@ -1044,12 +1045,34 @@ RSpec.describe TodoService do
       should_not_create_todo(user: skipped, target: noteable, action: Todo::DIRECTLY_ADDRESSED)
     end
 
-    it 'does not create a todo if user was already mentioned and todo is pending' do
+    context 'users already have pending todos and the multiple_todos feature is off' do
+      before do
         stub_feature_flags(multiple_todos: false)
+      end
 
-      create(:todo, :mentioned, user: member, project: project, target: noteable, author: author)
+      let_it_be(:pending_todo_for_member) { create(:todo, :mentioned, user: member, project: project, target: noteable) }
+      let_it_be(:pending_todo_for_guest) { create(:todo, :mentioned, user: guest, project: project, target: noteable) }
+      let_it_be(:pending_todo_for_admin) { create(:todo, :mentioned, user: admin, project: project, target: noteable) }
+      let_it_be(:note_mentioning_1_user) do
+        create(:note, project: project, note: "FYI #{member.to_reference}", noteable: noteable)
+      end
+
+      let_it_be(:note_mentioning_3_users) do
+        create(:note, project: project, note: 'FYI: ' + [member, guest, admin].map(&:to_reference).join(' '), noteable: noteable)
+      end
 
-      expect { service.update_note(note, author, skip_users) }.not_to change(member.todos, :count)
+      it 'does not create a todo if user was already mentioned and todo is pending' do
+        expect { service.update_note(note_mentioning_1_user, author, skip_users) }.not_to change(member.todos, :count)
+      end
+
+      it 'does not create N+1 queries for pending todos' do
+        # Excluding queries for user permissions because those do execute N+1 queries
+        allow_any_instance_of(User).to receive(:can?).and_return(true)
+
+        control_count = ActiveRecord::QueryRecorder.new { service.update_note(note_mentioning_1_user, author, skip_users) }.count
+
+        expect { service.update_note(note_mentioning_3_users, author, skip_users) }.not_to exceed_query_limit(control_count)
+      end
     end
 
     it 'does not create a todo if user was already mentioned and todo is done' do