title: Rails check if field value is the default value for that field
body: <p>I have a field named <code>number</code> on the <code>Address</code> model, which has a default of <code>0</code> set at database level. If I do <code>Address.new.number</code>, I will get <code>0</code> back.</p>
  
  <p>Now, I want to trigger a <code>before_create</code> callback only if <code>number != &lt;default value&gt;</code>. So, I will have <code>before_create :callback, unless: &lt;condition&gt;</code>. How can I check this using some Rails "logic"? I know I can compare to <code>0</code>, but I'd rather compare to a dynamical value, as retrieved by Rails.</p>