Validating referential integrity of a polymorphic association

Question

Working in a Rails app, I'm changing an optional belongs_to association to be polymorphic. The existing code was validating the referential integrity of the association, and I'm trying to figure out how to translate that into the polymorphic idiom.

A simplified but illustrative example...

The old:

class Climb < ActiveRecord::Base

  belongs_to :ladder
  validate   :ladder_exists_if_id_set

  def ladder_exists_if_id_set
    if ladder_id.present? && Ladder.find_by_id(ladder_id).blank?
      errors.add('Ladder id', 'invalid. Set a different ladder id or nil.')
    end
  end

end

The new:

class Climb < ActiveRecord::Base

  belongs_to :climbable, polymorphic: true
  validate   :climbable_exists_if_set

  def climbable_exists_if_set
    return unless climbable_id.present? && climbable_type.present?

    klass = climbable_type.constantize

    if klass.find_by_id(climbable_id).blank?
      errors.add("#{klass} id", "invalid. Set a different #{klass} id or nil.")
    end
  end

end

This seems to work, but is it the right approach? It seems like I'm reimplementing the dynamic getter/setter methods that polymorphism gives you.

Sidenote : I'm able to ignore the case where only _type or _id is set because Rails invalidates the record in that case.

Answer 1

I am wondering, why were you validating uniqueness of id in this way in your old code on the first place? Isn't it easier to write validates :ladder_id, uniqueness: true, allow_nil: true [, allow_blank: true] (you should decide what you want to allow) and with a custom message if you need one. Then see if Rails takes care of polymorphic association for you in your new code. I think you don't need your climbable_exists_if_set for that.

After your comment

It is probably even easier. You just have to validate presence of a ladder object. Note, not a ladder_id, but an object itself, so your validation should be as following:

validates :ladder, presence: true, allow_nil: true [, allow_blank: true]

In this case, rails validates that an actual ladder record with a given id exists in your ladders table.