Polymorphic relationships supporting referential integrity.

better_polymorphic.md

Usage:

  polymorphic :commentable do
    belongs_to :post
    belongs_to :comment
    belongs_to :product
  end

Adds validations that only one is set, and an accessor ('commentable' from above) that gets and sets the polymorphic relation.

better_polymorphic.rb

module BetterPolymorphic
  def polymorphic(polymorphic_name)
    reflections_prior = self.reflections

    yield

    # Hash subtraction
    new_reflections = (self.reflections.to_a - reflections_prior.to_a).
                      inject({}) {|acc, (k, v)| acc[k] = v; acc}

    define_method polymorphic_name do
      new_reflections.each_pair do |method, reflection|
        return send(method) if send(reflection.foreign_key)
      end
    end

    define_method "#{polymorphic_name}=" do |model|
      new_reflections.each_pair do |method, reflection|
        if model.is_a?(reflection.class_name.constantize)
          return send("#{method}=", model)
        end
      end
    end

    define_method "validate_#{polymorphic_name}_belongs_to_one_model" do
      if new_reflections.map(&:last).select {|r| send(r.foreign_key)}.size < 1
        errors.add(:base, 'must belong to a model')
      end
    end
    private "validate_#{polymorphic_name}_belongs_to_one_model"

    define_method "validate_#{polymorphic_name}_only_belongs_to_one_model" do
      if new_reflections.map(&:last).select {|r| send(r.foreign_key) }.size > 1
        errors.add(:base, 'cannot belong to more than one model')
      end
    end
    private "validate_#{polymorphic_name}_only_belongs_to_one_model"

    validate "validate_#{polymorphic_name}_belongs_to_one_model"
    validate "validate_#{polymorphic_name}_only_belongs_to_one_model"
  end
end

better_polymorphic_shared_examples.rb

shared_examples_for :better_polymorphic do |options|
  let(:polymorphic_name)      { options.keys.first   }
  let(:polymorphic_relations) { options.values.first }

  describe 'validation' do
    it 'fails if it does not belong to one association' do
      polymorphic_relations.each do |relation|
        subject.send("#{relation}=", nil)
      end

      subject.should have(1).error_on(:base)
    end

    it 'fails if it belongs to more than one association' do
      polymorphic_relations[0..1].each do |relation|
        subject.send("#{relation}=", Factory.build_stubbed(relation))
      end

      subject.should have(1).error_on(:base)
    end
  end

  describe "##{options.keys.first}" do
    it 'returns the model referenced by the real association' do
      polymorphic_relations.each do |relation|
        subject.send("#{relation}=", nil)
      end

      relation = polymorphic_relations.first
      model = Factory.build_stubbed(relation)
      subject.send("#{relation}=", model)
      subject.send(polymorphic_name).should == model
    end
  end

  describe "##{options.keys.first}=" do
    it 'sets the object to the real association' do
      polymorphic_relations.each do |relation|
        subject.send("#{relation}=", nil)
      end

      relation = polymorphic_relations.first
      model = Factory.build_stubbed(relation)
      subject.send("#{polymorphic_name}=", model)
      subject.send(relation).should == model
    end
  end
end