N+1 in has_many :through - ruby-on-rails

Question

I ran into problem N + 1
in association :
class Category < ApplicationRecord
has_many :categories_designs, dependent: :destroy
has_many :designs, through: :categories_designs
has_many :templates, ->{ where(is_template: true) }, through: :categories_designs, class_name: 'Design', source: :design
def marked_designs_as_new?
designs.select(:mark_design_as_new_until).where("mark_design_as_new_until >= ?", Time.now.in_time_zone.beginning_of_day).exists?
end
end
And I want to use the marked_designs_as_new? method in the view.
- #categories.each do |category|
= category.title.titleize
- if category.marked_designs_as_new?
.design-type-marked
NEW
In my controller I call:
#categories = Category.includes(categories_designs: :design).visible
And I'm faced with the problem of N + 1.
Category Load (0.4ms) SELECT "categories".* FROM "categories" WHERE "categories"."hidden" = $1 ORDER BY "categories"."position" ASC LIMIT $2 OFFSET $3 [["hidden", false], ["LIMIT", 100], ["OFFSET", 0]]
CategoriesDesign Load (0.4ms) SELECT "categories_designs".* FROM "categories_designs" WHERE "categories_designs"."category_id" IN (1, 3, 4, 5, 6, 7, 8)
Design Load (0.5ms) SELECT "designs".* FROM "designs" WHERE "designs"."id" IN (1, 4, 3, 6)
(0.7ms) SELECT COUNT(*) FROM "designs" INNER JOIN "categories_designs" ON "designs"."id" = "categories_designs"."design_id" WHERE "categories_designs"."category_id" = $1 AND "designs"."is_template" = $2 [["category_id", 1], ["is_template", true]]
Design Exists (0.7ms) SELECT 1 AS one FROM "designs" INNER JOIN "categories_designs" ON "designs"."id" = "categories_designs"."design_id" WHERE "categories_designs"."category_id" = $1 AND (mark_design_as_new_until >= '2018-03-13 00:00:00') LIMIT $2 [["category_id", 1], ["LIMIT", 1]]
(0.5ms) SELECT COUNT(*) FROM "designs" INNER JOIN "categories_designs" ON "designs"."id" = "categories_designs"."design_id" WHERE "categories_designs"."category_id" = $1 AND "designs"."is_template" = $2 [["category_id", 3], ["is_template", true]]
............. etc.
why?

Answer 1

Ok, your .select(:mark_design_as_new_until) performs another query to the database. What you should do is use an array select method in the following way:
.select(&:mark_design_as_new_until)
This gives you an array of designs loaded in the memory on which you can perform .any? method to check your condition:
.select(&:mark_design_as_new_until).any? { |design| design.mark_design_as_new_until >= Time.now.in_time_zone.beginning_of_day }
And of course, include designs in your Category.
Category.includes(:designs, ...)

Answer 2

Did you try Category.includes([:categories_designs, :design]) Also, you can change the marked_designs_as_new? method as follows,
def marked_designs_as_new?
designs.select{ |x| x.marked_designs_as_new? }.any?
end
design.rb
class Design
def marked_designs_as_new?
mark_design_as_new_until >= Time.now.in_time_zone.beginning_of_day
end
end