title: Error : CoreData: error: Failed to call designated initializer on NSManagedObject class 'Product'
body: <p>I start to use core data with ios 5. I have my product model :</p>
  
  <p>Product.m :</p>
  
  <pre><code>#import "Product.h"
  @implementation Product
      @dynamic category_id;
      @dynamic label;
      @dynamic price;
  @end
  </code></pre>
  
  <p>Product.h :</p>
  
  <pre><code>#import &lt;Foundation/Foundation.h&gt;
  #import &lt;CoreData/CoreData.h&gt;
  
  @interface Product : NSManagedObject 
    @property (nonatomic, retain) NSString *category_id;
    @property (nonatomic, retain) NSString *label;
    @property (nonatomic, retain) NSString *price;
  @end
  </code></pre>
  
  <p>I try to parse an xml with a custom class using NSXMLParserDelegate. My xml looks like : </p>
  
  <pre><code>&lt;section id="2"&gt;
    &lt;label&gt;Animaux&lt;/label&gt;
    &lt;image&gt;Images/Rayons/Bandeau/Animaux.png&lt;/image&gt;
    &lt;key&gt;Images/Rayons/Bandeau/Animaux.png&lt;/key&gt;
    &lt;products&gt;
       &lt;Product id="21"&gt;
          &lt;category_id&gt;Chat&lt;/category_id&gt;
          &lt;label&gt;Aliments pour chat&lt;/label&gt;
          &lt;price&gt;2.00&lt;/price&gt;
       &lt;/Product&gt;
       &lt;Product id="1286"&gt;
          &lt;category_id&gt;Chat&lt;/category_id&gt;
          &lt;label&gt;Boite de paté&lt;/label&gt;
          &lt;price&gt;0.00&lt;/price&gt;
       &lt;/Product&gt;
    &lt;/products&gt;
  &lt;/sections&gt;
  </code></pre>
  
  <p>When i have a balise Product, I build my model like this : </p>
  
  <pre><code>item = [[NSClassFromString(className) alloc] init];
  </code></pre>
  
  <p>And when i have a property as category_id, i do : </p>
  
  <pre><code>[item setValue:currentNodeContent forKey:elementName];
  </code></pre>
  
  <p>And i got the error : </p>
  
  <p><strong>CoreData: error: Failed to call designated initializer on NSManagedObject class 'Product'</strong> </p>
  
  <p>Any idea?</p>