class User
  validates_uniqueness_of   :login, :case_sensitive => false
end

describe User do

  def valid_user_attributes
     { :login => 'quire', :email => 'quire@howcast.com', :password => 'quire', :password_confirmation => 'quire'}
  end

  def create_user(options = {})
      user = User.create(valid_user_attributes.merge(options))
  end

  it 'should be invalid if login is not unique' do
    john = create_user :login => "john"
    assert_no_difference User, :count do
      john_smith = create_user :login => "john"
      john_smith.errors.invalid?(:login).should_not be_nil
    end
  end
  ...
end