title: CodeIgniter DataMapper Error: You must use the "set" method to update an entry
body: <p>I'm using codeigniter/datamapper to develop an inviocing application and I'm getting an error that i don't understand. If I do the following:</p>
  
  <pre><code>$i = new Invoice();    
  $i-&gt;save();
  </code></pre>
  
  <p>Then i get the following error:
      A Database Error Occurred</p>
  
  <pre><code>You must use the "set" method to update an entry.
  
  Filename: /Users/jim/Sites/example.com/libraries/Datamapper.php
  
  Line Number: 1635
  </code></pre>
  
  <p>However I can run this all day without any problem:</p>
  
  <pre><code>$i = new Invoice();    
  $i-&gt;notes ='x';
  $i-&gt;save();
  </code></pre>
  
  <p>Just wondered why it behaves like this? Any help/comments much appreciated. Thanks</p>