example parent child

parent_child.rb

# categories without link
rails g model Category name parent_id:integer:index

class Category < ApplicationRecord
  has_many :subcategories, class_name: 'Category', foreign_key: 'parent_id', dependent: :destroy
  belongs_to :parent, class_name: 'Category', foreign_key: 'parent_id', optional: true
end

# linked_categories
rails g model Category name
rails g model LinkedCategory parent_id:integer child_id:integer

class Category < ApplicationRecord
  has_many :linked_categories, foreign_key: 'parent_id'
  has_many :subcategories, through: :linked_categories, source: :child
  has_many :linked_me, class_name: 'LinkedCategory', foreign_key: 'child_id'
  has_many :linked_me_categories, through: :linked_me, source: :parent

  def self.main_categories
    ids = all.ids - joins(:linked_me).ids
    where(id: ids)
  end

  def self.display_subcategories(el, root_name)
    if el.subcategories.present?
      el.subcategories.each_with_object([]) do |el, arr|
        if el.subcategories.size <= 1
          arr << { root: root_name, subcategories: el.name }
        else
           arr << display_subcategories(el, root_name)
        end
      end
    end
  end
end

class LinkedCategory < ApplicationRecord
  belongs_to :parent, class_name: 'Category', foreign_key: 'parent_id'
  belongs_to :child, class_name: 'Category', foreign_key: 'child_id'
end