Question

我有自引用Category模型及相关产品。

class Category < ApplicationRecord
  has_many :subcategories,
           class_name: 'Category',
           foreign_key: 'parent_id'
  has_many :products
  scope :main, -> { where(parent_id: nil) }
end

为了获得所有主要类别的子类别和产品的数量，想要执行双左外连接。

这是我不使用left_outer_joins方法的方法。

Category.main
  .select('categories.*, COUNT(DISTINCT cats.id) as sub_count, COUNT(products.id) as prod_count')
  .joins('LEFT JOIN categories AS cats ON categories.id = cats.parent_id')
  .joins('LEFT JOIN products ON cats.id = products.category_id')
  .group('categories.name')

我想用left_outer_joins方法做同样的事情，因为从代码角度看它更清晰。

Category.main
  .select('categories.name, COUNT(DISTINCT subcategories_categories.id) as sub_count, COUNT(products.id) as prod_count')
  .left_joins(:subcategories, :products)
  .group('categories.name')

输出以下查询：

SELECT categories.name, COUNT(DISTINCT subcategories_categories.id) as sub_count, COUNT(products.id) as prod_count 
FROM "categories" 
LEFT OUTER JOIN "categories" "subcategories_categories" ON "subcategories_categories"."parent_id" = "categories"."id" 
LEFT OUTER JOIN "products" ON "products"."category_id" = "categories"."id" 
WHERE "categories"."parent_id" IS NULL 
GROUP BY categories.name

正如您所看到的那样，问题出现在第二个LEFT OUTER JOIN，因为它引用了categories而不是subcategories_categories。

对于此类方案，是否有left_outer_joins的解决方法？

Answer 1

尝试left_outer_joins(subcategories: :products)

ActiveRecord left_outer_joins在自引用表

1 个答案: