def available_parents
      if persisted?
        unavailable_ids = ancestors.map(&:id)
        unavailable_ids << id
      end

      Page.published.where.not(id: unavailable_ids).order(:title)
    end
def available_parents
      if persisted?
        unavailable_ids = ancestors.map(&:id)
        unavailable_ids << id
      end

      Page.published.where.not(id: unavailable_ids).order(:title)
    end
Return all available parents
 TODO: Update parent behaviour
 What happens when parent is deleted or goes from published to
 draft. Possibly allow it but show warnings on the dashboard.