title: Where is the right place to put _changed? methods in Rails?
body: <p>I have a user model that checks to see if a value has changed before_save (by running User.zipcode_changed?).  The idea is that this will queue up a delayed job if it has.</p>
  
  <p>Problem is, when I migrate the app from scratch I get an error:</p>
  
  <pre><code>An error has occurred, all later migrations canceled:
  
  undefined method `postcode_changed?' for #&lt;User:0x105db6158&gt;
  </code></pre>
  
  <p>Therefore, where should I be putting these?  Is the model the wrong place?</p>