Testing Validations using RSpec

I just ran into an interesting issue while testing a Rails application with RSpec.  A spec with the following line in it was failing:

    ar2.should_not be_valid

Here ar2 is a model that was constructed in a way that violated a logical constraint.  I attempted to make this spec pass by adding a custom validation method to the model as such:

class Registration < ActiveRecord::Base
  validate :fields_match

  private

  def fields_match
    return true if model2.nil?
    model2.model1.id == model1.id
  end
end

All field names have had their names changed to protect their identity.

This didn't work.  As it turns out merely returning false from the validation method isn't enough to mark a model as invalid.  This answer on StackOverflow helped identify the problem as not adding an error to the list of validation errors for the model.

An updated version where I add an error finally made everything pass:

class Registration < ActiveRecord::Base
  validate :fields_match

  private

  def fields_match
    return true if model2.nil?
    return true if model2.model1.id == model1.id
    errors.add(:model2_id, "model2 doesn't match model1")
  end
end