You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: knowledge_area_lesson_plan.rb
    path of file: ./repos/i-diario/app/models
    
    the code of the file until where you have to start completion: class KnowledgeAreaLessonPlan < ActiveRecord::Base
  include Audit
  include Filterable
  in
    
  def valid_for_destruction?
    @valid_for_destruction if defined?(@valid_for_destruction)
    @valid_for_destruction = begin
      lesson_plan.valid?
      forbidden_error = I18n.t('errors.messages.not_allowed_to_post_in_date')
      if lesson_plan.errors[:start_at].include?(forbidden_error) || lesson_plan.errors[:end_at].include?(forbidden_error)
        errors.add(:base, forbidden_error)
        false
      else
        true
      end