Rails 4: 查找所有 has_many 中没有 parents 的条目 :though self-referential join

Question

所以我的设置是我有类别，一个 category 可以有很多 parents 和很多 children 通过 self-referential 加入 table category_relationships。使用下面的内容，这一切在创建和检索这些关系时都非常有效。

不过我现在想做的是，找到没有任何 parents 的类别（基本上是所有 top-level 类别）。

我已经尝试了几种不同的 where.not 类型实现，但就是想不出正确的表达式。

category.rb

# == Schema Information
#
# Table name: categories
#
#  created_at :datetime         not null
#  id         :integer          not null, primary key
#  name       :string(255)      not null
#  updated_at :datetime         not null
#

class Category < ActiveRecord::Base
  # CategoryRelationships (Parents & Children)
  # ==========================================================================================================  
    has_many                        :parent_child_relationships,
                                      class_name:     "CategoryRelationship",
                                      foreign_key:    :child_id, 
                                      inverse_of:     :parent,
                                      dependent:      :destroy

    has_many                        :parents,
                                      through:        :parent_child_relationships,
                                      source:         :parent

    has_many                        :child_parent_relationships,
                                      class_name:     "CategoryRelationship",
                                      foreign_key:    :parent_id,
                                      inverse_of:     :child,
                                      dependent:      :destroy

    has_many                        :children,
                                      through:        :child_parent_relationships, 
                                      source:         :child

end

category_relationships.rb

# == Schema Information
#
# Table name: category_relationships
#
#  child_id   :integer          not null
#  created_at :datetime         not null
#  id         :integer          not null, primary key
#  parent_id  :integer          not null
#  updated_at :datetime         not null
#

class CategoryRelationship < ActiveRecord::Base
  # Parent (Category)
  # ========================================================================================================== 
    belongs_to                      :parent,
                                      class_name:     "Category",
                                      inverse_of:     :parent_child_relationships

  # Child (Category)
  # ========================================================================================================== 
    belongs_to                      :child,
                                      class_name:     "Category",
                                      inverse_of:     :child_parent_relationships

  # Validations
  # ========================================================================================================== 
    validates                       :parent, 
                                      presence:       true

    validates                       :child, 
                                      presence:       true

end

Answer 1

我最终选择了：

Category.joins("LEFT OUTER JOIN category_relationships ON categories.id = category_relationships.child_id").where("category_relationships.child_id IS NULL")

不过，我不确定它的效率如何，也不确定是否有更好的方法，因为我听说 LEFT OUTER JOIN 在较大的数据集上可能成本很高。

Rails 4: 查找所有 has_many 中没有 parents 的条目 :though self-referential join

Rails 4: Find all entries with no parents in has_many :though self-referential join

ruby-on-rails

has-many-through

self-reference