Custom Rails Validator

app_helpers_my_model_validator.rb

# app/helpers/my_model_helper.rb
class MyModelValidator < ActiveModel::Validator

  def validate(mymodel)
     unless mymodel.starts_at.nil? || mymodel.ends_at.nil?
       validate_times(mymodel)
     end
  end

  def validate_times(mymodel)
    unless mymodel.starts_at < mymodel.ends_at
      event.errors[:end_date] << 'must be after start date'
    end
  end

end

app_models_my_model.rb

# app/models/my_model.rb
class MyModel < ActiveRecord::Base
  include ActiveModel::Validations
  validates_with MyModelValidator
end