Rails 包括类别和子类别模型

Question

如何包含类别（“父”）和子类别（“子”）以摆脱 N+1 查询

class Category < ApplicationRecord
  belongs_to :parent, class_name: 'Category', foreign_key: :parent_id
  has_many :children, class_name: 'Category', foreign_key: :parent_id

  has_many :article_categories
  has_many :articles, through: :article_categories

  scope :roots, -> { where(parent_id: 0) }
  scope :children, -> { where.not(parent_id: 0) }
end

parent_id 为 0 的类别是“Category/Parent”

parent_id != 0 的类别是“子类别/儿童”

我在控制器中声明实例：

@articles = articles.includes(:categories)

在视图中：

@articles.each do |article|
  article.categories.roots #N+1 query solved using "includes(:categories)"
  article.categories.children.first  #N+1 query need to solve
  ..............

问题是因为article.categories.children.first，每一个新的循环都会对数据库产生一个新的请求

N+1 个请求是：

Category Load (0.8ms)  SELECT  "categories".* FROM "categories" INNER JOIN "article_categories" ON "categories"."id" = "article_categories"."category_id" WHERE "article_categories"."article_id" = $1 AND ("categories"."parent_id" != $2) ORDER BY "categories"."id" ASC LIMIT $3  [["article_id", 450], ["parent_id", 0], ["LIMIT", 1]]

我需要包含“父/类别”

还包括“children/Subcategories”以摆脱来自 ...“children.first”的 N+1 查询

更多细节：

articles.includes(:categories) =>

Article Load (3.9ms)  SELECT "articles".* FROM "articles" WHERE "articles"."customer_type" = $1 AND "articles"."aasm_state" IN ('published', 'unpublished') ORDER BY "articles"."title" ASC  [["customer_type", 0]]
ArticleCategory Load (3.6ms)  SELECT "article_categories".* FROM "article_categories" WHERE "article_categories"."article_id" IN (1, 2, ...)
Category Load (0.7ms)  SELECT "categories".* FROM "categories" WHERE "categories"."id" IN (1, 15, ...)

Answer 1

我总是用Bullet gem来摆脱N+1问题

Answer 2

由于您要预加载文章的子类别，您应该在Article 模型中定义此关联：

class Article < ApplicationRecord
  has_many :article_categories

  # '-> { children }' executes 'children' scope defined in 'Category' model
  has_many :children_categories, -> { children }, through: :article_categories, source: :category, class_name: "Category"
end

现在您可以使用此关联预加载和获取子类别：

# in controller
@articles = articles.includes(:children_categories)

# in view
@articles.each do |article|
  article.children_categories.first

编辑

您可能还想将default_scope 添加到类别模型以始终仅获取子类别：

class Category < ApplicationRecord
  default_scope { children }
end

如果您这样做，则不必将-> { children } 条件添加到关联中。