title: can't convert nil into an exact number
body: <p>I want users to be able to comment on others' microposts but I keep getting the following error: can't convert nil into an exact number</p>
  
  <p>It is coming from the timestamp in the View/comments/_form file below. For some reason @comment.created_at comes back as nil</p>
  
  <p>View/comments/_form:(this partial is called at the end of every micropost)</p>
  
  <pre><code>&lt;span class="content"&gt;&lt;%= @comment.content %&gt;&lt;/span&gt;
  &lt;span class="timestamp"&gt;Said &lt;%= time_ago_in_words(@comment.created_at) %&gt; ago.&lt;/span
  &lt;%= form_for(@comment) do |f| %&gt;
    &lt;%= f.text_field :content, placeholder: "Say Something..." if signed_in? %&gt;
  &lt;% end %&gt;
  </code></pre>
  
  <p>User Model:</p>
  
  <pre><code>attr_accessible :name, :email, :password, :password_confirmation #is this secure with password there?
  attr_protected :admin   #attr_protected necessary?
  has_many :microposts, dependent: :destroy
  has_many :comments, :through =&gt; :microposts, dependent: :destroy
  </code></pre>
  
  <p>Micropost Model:</p>
  
  <pre><code>attr_accessible :comment #basically the content of the post
  attr_protected :user_id
  has_many :comments, dependent: :destroy
  </code></pre>
  
  <p>Comment Model:</p>
  
  <pre><code>attr_accessible :content, :micropost
  belongs_to :user
  belongs_to :micropost
  validates :user_id, presence: true
  validates :micropost_id, presence: true
  validates :content, presence: true
  default_scope order: 'comments.created_at ASC'   #is this necessary?
  </code></pre>
  
  <p>Comments Controller:</p>
  
  <pre><code>def create
    @micropost = Micropost.find_by_id(params[:id])   #is this necessary?
    @comment = current_user.comments.create(:micropost =&gt; @micropost)
    redirect_to :back
  end
  </code></pre>
  
  <p>User Controller:</p>
  
  <pre><code>def show
    @user = User.find_by_id(params[:id])
    @microposts = @user.microposts.paginate(page: params[:page])
    @micropost  = current_user.microposts.build
    @comments = @micropost.comments
    @comment = current_user.comments.create(:micropost =&gt; @micropost) #build, new or create??
  end
  </code></pre>
  
  <p>Routes:</p>
  
  <pre><code>resources :users 
  resources :microposts, only: [:create, :destroy] 
  resources :comments, only: [:create, :destroy]
  </code></pre>
  
  <p>SQL:</p>
  
  <p>"comment"=>{"content"=>"EXAMPLE"}}
    User Load (0.8ms)  SELECT "users".* FROM "users" WHERE "users"."remember_token" = 'H09yZpAv5qhmT3ok5fXfnQ' LIMIT 1
    Micropost Load (0.7ms)  SELECT "microposts".* FROM "microposts" WHERE "microposts"."id" IS NULL </p>