attribute_matchers.rb

# spec/support/matchers/attribute.rb
module AttributeMatchers

  Spec::Matchers.define :have_only_attributes do |attributes|
    match do |target|
      actual_attributes = target.attribute_names.map(&:to_sym)
      diff = ((actual_attributes | attributes) - (actual_attributes & attributes))
      diff.length == 0
    end
  end
end

# spec/spec_helper
Dir["#{File.dirname(__FILE__)}/support/**/*.rb"].each { |file| require file }

Spec::Runner.configure do |config|
  config.include AttributeMatchers
end

# app/models/user.rb
class User < ActiveRecord::Base
  def self.method_returning_only_one_user
    first(:select => "first_name, last_name, email")
  end
end

# spec/models/user_spec.rb
User.method_returning_only_one_user.should have_only_attributes([:first_name, :last_name, :email])