Comparison of has_many :through with writing a method

example.rb

class Product < ActiveRecord::Base
  has_many :category_products, dependent: :delete_all
  has_many :categories, through: :category_products, ->{ order(:name) }
end

class CategoryProduct < ActiveRecord::Base
  belongs_to :category
  belongs_to :product
end

class Category < ActiveRecord::Base
  has_many :category_products, dependent: :delete_all, ->{ order(:position) }
  has_many :products, through: :category_products
end

# category.products generates 1 query:
#
# SELECT products.* FROM products
# INNER JOIN category_products
# ON products.id = category_products.product_id
# WHERE category_products.category_id = ?
# ORDER BY category_products.position

# vs.

class Product < ActiveRecord::Base
  has_many :category_products, dependent: :delete_all
end

class CategoryProduct < ActiveRecord::Base
  belongs_to :category
  belongs_to :product
end

class Category < ActiveRecord::Base
  has_many :category_products, dependent: :delete_all, ->{ order(:position) }

  def products
    category_products.preload(:product).map{ |c| c.product }
  end
end

# category.products generates 2 queries:
#
# SELECT * FROM category_products
# WHERE category_products.category_id = ?
# ORDER BY category_products.position
# 
# SELECT * FROM products WHERE id IN (?)