title: Build vs new in Rails 3
body: <p>In the Rails 3 <a href="http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html" rel="noreferrer">docs</a>, the <code>build</code> method for associations is described as being the same as the <code>new</code> method, but with the automatic assignment of the foreign key. Straight from the docs:</p>
  
  <pre><code>Firm#clients.build (similar to Client.new("firm_id" =&gt; id))
  </code></pre>
  
  <p>I've read similar elsewhere.</p>
  
  <p>However, when I use <code>new</code> (e.g. <code>some_firm.clients.new</code> without any parameters), the new client's <code>firm_id</code> association <strong>is</strong> automatically created. I'm staring at the results right now in the console!</p>
  
  <p>Am I missing something? Are the docs a bit out of date (unlikely)? What's the difference between <code>build</code> and <code>new</code>?</p>