Rails 4 找到缺少某些子关联的父关联

Question

我正在尝试查询哪些父（类别）记录没有具有给定属性（author.id）的 ha 子记录（post）。

class Category < ActiveRecord::Base
  has_many :posts, dependent: :destroy
end

class Post < ActiveRecord::Base
  belong_to :category
end

class Author < ActiveRecord::Base
  has_many :posts, dependent: :destroy
end

我正在尝试查找作者从未 post 编辑过哪些类别。我的想法是获取一个 GIVEN 作者没有 post 的类别数组，他们可以还有其他 posts...只是不是特定作者

@author_unused_categories = @author.unused_categories


class Author < ActiveRecord::Base

  def unused_categories
    categories = Categories.all
    author_used_categories = Post.where(author_id: self.id).select(:category).uniq ??????
  end
end

Answer 1

categories = Author.includes(posts: :category).select('category.id AS category_id').map(&:category_id)

unused_categories = Category.where.not(id: categories)

Rails 4 找到缺少某些子关联的父关联

Rails 4 find parent associations which are missing certain child associations

ruby-on-rails

associations

has-many