page.rb

class Page < ApplicationRecord
  extend FriendlyId
  friendly_id :meta_slug, use: :slugged
  has_one_attached :banner

  def related_pages
    pages_this_relates_to + pages_related_to_this
  end

  has_and_belongs_to_many :pages_this_relates_to,
    class_name: "Page",
    join_table: "related_pages",
    association_foreign_key: "other_page_id"

  has_and_belongs_to_many :pages_related_to_this,
    class_name: "Page",
    join_table: "related_pages",
    foreign_key: "other_page_id"

  has_and_belongs_to_many :related_pages,
    class_name: "Page",
    join_table: "related_pages",
    association_foreign_key: "other_page_id"
end

related_page.rb

class RelatedPage < ApplicationRecord
  has_and_belongs_to_many :pages
end