title: empty class field in RoR
body: <p>On the Rails Guide, I saw a class was defined as </p>
  
  <pre><code>class Person &lt; ActiveRecord::Base
  end
  </code></pre>
  
  <p>and then it wrote </p>
  
  <pre><code>&gt;&gt;p = Person.new(:name =&gt; "John Doe")
  =&gt;#&lt;Person id: nil, name: "John Doe", created_at: nil, :updated_at: nil
  </code></pre>
  
  <p>Since class Person has no field, why here it is valid to pass a :name attribute to the constructor? Shouldn't there be some error with it? </p>
  
  <p>Also, why id, name, created_at all have no colon but :updated_at has one? </p>