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）的子记录（帖子）。

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发布过的类别。我们的想法是获取一系列类别，这些类别没有给定作者的帖子，他们可以有其他帖子......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)