Avoid N+1 queries when loading submitted counts

Co-authored-by: Copilot <copilot@github.com>

Author: zetter-rpf

app/controllers/api/lessons_controller.rb

@@ -11,13 +11,15 @@ class LessonsController < ApiController
     def index
       accessible_lessons = filtered_lessons_scope
                            .accessible_by(current_ability)
-                           .includes(project: :remixes)
-      @lessons_with_users = accessible_lessons.with_users
 
       if current_user&.school_teacher?(school) || current_user&.school_owner?(school)
+        accessible_lessons = accessible_lessons.includes(project: { remixes: %i[submitted_school_projects] })
+        @lessons_with_users = accessible_lessons.with_users
         render :teacher_index, formats: [:json], status: :ok
       else
         remixes = user_remixes(accessible_lessons)
+        accessible_lessons = accessible_lessons.includes(project: :remixes)
+        @lessons_with_users = accessible_lessons.with_users
         @lessons_with_users_and_remixes = @lessons_with_users.zip(remixes)
         render :student_index, formats: [:json], status: :ok
       end

app/models/lesson.rb

@@ -35,7 +35,7 @@ def with_user
   def submitted_count
     return 0 unless project
 
-    project.remixes.count { |remix| remix.school_project&.submitted? }
+    project.remixes.sum { |r| r.submitted_school_projects.length }
   end
 
   private

app/models/project.rb

@@ -19,6 +19,7 @@ module Types
   has_many_attached :videos
   has_many_attached :audio
   has_one :school_project, dependent: :destroy
+  has_many :submitted_school_projects, -> { in_state(:submitted) }, class_name: 'SchoolProject'
 
   accepts_nested_attributes_for :components
   accepts_nested_attributes_for :scratch_component

spec/features/lesson/listing_lessons_spec.rb

@@ -86,14 +86,20 @@
   end
 
   it 'includes the submitted_count for each lesson' do
+    student_2 = create(:student, school:)
+    create(:class_student, school_class:, student_id: student_2.id)
+
     lesson.update!(school_class_id: school_class.id)
-    remix = create(:project, school:, remixed_from_id: lesson.project.id, user_id: student.id)
-    remix.school_project.transition_status_to!(:submitted, student.id)
+
+    [student, student_2].each do |student|
+      remix = create(:project, school:, remixed_from_id: lesson.project.id, user_id: student.id)
+      remix.school_project.transition_status_to!(:submitted, student.id)
+    end
 
     get("/api/lessons?school_class_id=#{school_class.id}", headers:)
     data = JSON.parse(response.body, symbolize_names: true)
 
-    expect(data.first[:submitted_count]).to eq(1)
+    expect(data.first[:submitted_count]).to eq(2)
   end
 
   context 'when filtering by project_identifier' do