rails-docstore/app/models/category.rb

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

  before_destroy :check_for_subcategories

  def get_fqcn
    fqcn = name
    cat_parent = parent

    while cat_parent != nil do
      fqcn = cat_parent.name + "/" + fqcn
      cat_parent = cat_parent.parent
    end

    fqcn
  end

  private
    def check_for_subcategories
      unless subcategories.empty?
        errors.add(:base, 'Subcategories present')
        throw :abort
      end
    end
end