Unexpected Results Testing validates :presence => true

When the below test is executed it fails on the last assert with <1>
expected but was <0>. How was save unsuccessful, but the count of the
associations afterward 0?

Thanks.

# app/model/user.rb
class User < ActiveRecord::Base

devise :database_authenticatable, :recoverable, :rememberable, :trackable, :validatable

attr_accessible :email, :password, :password_confirmation, :remember_me

  has_many :organization_roles
  has_many :roles, :through => :organization_roles
  validates :organization_roles, :presence => true
  accepts_nested_attributes_for :organization_roles

  def role?(role)
    return !!self.roles.find_by_name(role.to_s.camelize)
  end
end

# test/unit/user_test.rb
require 'test_helper'

class UserTest < ActiveSupport::TestCase

  test "cannot remove all roles" do
    user = users(:one)
    assert_equal 1, user.organization_roles.count
    user.organization_roles.clear
    assert_false user.save
    assert_false user.errors[:organization_roles].blank?
    assert_equal 1, user.reload.organization_roles.count
  end

end