def pbt_parent
      val = pbt
      if val && !pbt_id.nil?
        if poly?
          "#{pbt_type}".constantize.where(id: pbt_id).first
        else
          "#{val}".camelize.constantize.where(id: pbt_id).first
        end
      end
    end
def pbt_parent
      val = pbt
      if val && !pbt_id.nil?
        if poly?
          "#{pbt_type}".constantize.where(id: pbt_id).first
        else
          "#{val}".camelize.constantize.where(id: pbt_id).first
        end
      end
    end
Get the parent relation.  Polymorphic relations are prioritized first.
 @return [Object, nil] ActiveRecord object instasnce