def pbt_parents
      if poly?
        Array[pbt_parent].compact
      else
        self.class.pbts.map do |i|
          try{ "#{i}".camelize.constantize.where(id: send("#{i}_id")).first }
        end.compact
      end
    end
def pbt_parents
      if poly?
        Array[pbt_parent].compact
      else
        self.class.pbts.map do |i|
          try{ "#{i}".camelize.constantize.where(id: send("#{i}_id")).first }
        end.compact
      end
    end
All belongs_to parents as class objects. One if polymorphic.
 @return [Array<Object>] ActiveRecord classes of parent objects.