Remove N+1 query for project and group boards

- Add test for N+1 queries
- Add destroyable lists scope to Board and List
- Preload lists for both project and group boards

app/models/board.rb

@@ -5,10 +5,13 @@ class Board < ApplicationRecord
   belongs_to :project
   has_many :lists, -> { order(:list_type, :position) }, dependent: :delete_all # rubocop:disable Cop/ActiveRecordDependent
+  has_many :destroyable_lists, -> { destroyable }, class_name: "List"
   validates :project, presence: true, if: :project_needed?
   validates :group, presence: true, unless: :project
+  scope :with_associations, -> { preload(:destroyable_lists) }
+
   def project_needed?
     !group
   end

app/serializers/board_serializer.rb

@@ -2,4 +2,12 @@
 class BoardSerializer < BaseSerializer
   entity BoardSimpleEntity
+
+  def represent(resource, opts = {})
+    if resource.respond_to?(:with_associations)
+      resource = resource.with_associations
+    end
+
+    super
+  end
 end

lib/api/boards.rb

@@ -27,7 +27,7 @@ module API
         end
         get '/' do
           authorize!(:read_board, user_project)
-          present paginate(board_parent.boards), with: Entities::Board
+          present paginate(board_parent.boards.with_associations), with: Entities::Board
         end
         desc 'Find a project board' do

lib/api/boards_responses.rb

@@ -11,7 +11,7 @@ module API
         end
         def board_lists
-          board.lists.destroyable
+          board.destroyable_lists
         end
         def create_list

lib/api/entities.rb

@@ -1101,7 +1101,7 @@ module API
       expose :project, using: Entities::BasicProjectDetails
       expose :lists, using: Entities::List do |board|
-        board.lists.destroyable
+        board.destroyable_lists
       end
     end

lib/api/group_boards.rb

@@ -37,7 +37,7 @@ module API
           use :pagination
         end
         get '/' do
-          present paginate(board_parent.boards), with: Entities::Board
+          present paginate(board_parent.boards.with_associations), with: Entities::Board
         end
       end

spec/support/api/boards_shared_examples.rb

@@ -14,6 +14,16 @@ shared_examples_for 'group and project boards' do |route_definition, ee = false|
     end
   end
+  it 'avoids N+1 queries' do
+    pat = create(:personal_access_token, user: user)
+    control = ActiveRecord::QueryRecorder.new { get api(root_url, personal_access_token: pat) }
+
+    create(:milestone, "#{board_parent.class.name.underscore}": board_parent)
+    create(:board, "#{board_parent.class.name.underscore}": board_parent)
+
+    expect { get api(root_url, personal_access_token: pat) }.not_to exceed_query_limit(control)
+  end
+
   describe "GET #{route_definition}" do
     context "when unauthenticated" do
       it "returns authentication error" do