with_slug.rb

module WithSlug
  extend ActiveSupport::Concern

  included do
    validates :slug,
              uniqueness: { scope: :university_id }
    validates :slug,
              format: { with: /\A[a-z0-9\-]+\z/, message: I18n.t('slug_error') }

    before_validation :regenerate_slug

    def regenerate_slug
      current_slug = self.slug
      n = 0
      while slug_unavailable?(self.slug)
        n += 1
        self.slug = [current_slug, n].join('-')
      end
    end

    protected

    def slug_unavailable?(slug)
      self.class.unscoped.where(university_id: self.university_id, slug: slug).where.not(id: self.id).exists?
    end
  end
end