マイクロポスト作成エラー

<h2>問題</h2>
テストで以下のようにmicropostをbuildしたら自動的にuser_idにuser.idが入っているはずなのにnil

<h2>原因？</h2>
userの生成ができていないか、属性がおかしい？

<h2>テスト</h2>
spec/models/micropost_spec.rb
<pre><code>
  let(:user) { FactoryGirl.create(:user) } #この行からおかしい？
  before do
    @micropost = user.microposts.build(content: "Lorem ipsum")
  end

  subject { @micropost }

  it { should respond_to(:content) }
  it { should respond_to(:user_id) }
  it { should respond_to(:user) }
  its(:user) { should eq user}

  it { should be_valid }
</pre></code>

<h2>エラー</h2>
<pre><code>
1) Micropost 
     Failure/Error: it { should be_valid }
       expected #<Micropost id: nil, content: nil, user_id: 181, created_at: nil, updated_at: nil> to be valid, but got errors: Content can't be blank
     # ./spec/models/micropost_spec.rb:16:in `block (2 levels) in <top (required)>'
</code></pre>

<h2>ユーザのモデル</h2>
spp/models/user.rb (<は入れるとおかしくなるのでextendsで代用)
<pre><code>
class User extends ActiveRecord::Base
	has_many :microposts, dependent: :destroy
	attr_accessible :name, :email, :password, :password_confirmation
	before_save { self.email = email.downcase }
	before_create :create_remember_token
	validates :name, presence: true, length: { maximum: 50 }
	VALID_EMAIL_REGEX = /\A[\w+\-.]+@[a-z\d\-.]+\.[a-z]+\z/i
	validates :email, presence: true, format: { with: VALID_EMAIL_REGEX },
		uniqueness: { case_sensitive:false }
	has_secure_password
	validates :password, length: { minimum: 6 }

	def User.new_remember_token
		SecureRandom.urlsafe_base64
	end

	def User.encrypt(token)
		Digest::SHA1.hexdigest(token.to_s)
	end

	private

		def create_remember_token
			self.remember_token = User.encrypt(User.new_remember_token)
		end
end

</code></pre>
<h2>マイクロポストのモデル</h2>
app/models.micropost.rb
<pre><code>
class Micropost extends ActiveRecord::Base
  belongs_to :user
  default_scope -> { order('created_at DESC') }
  validates :user_id, presence: true
  validates :content, presence: true, length: { maximum: 140 }
end
</code></pre>