title: Django dump : models don't validate
body: <p>When I try to dump the data of my model in Django with this instruction :  </p>
  
  <pre><code>python manage.py dumpdata app&gt; temp_data.json
  </code></pre>
  
  <p>It gives the following error :</p>
  
  <pre><code>Error: One or more models did not validate:
  asset.authpermission: "codename": CharField cannot have a "max_length" greater than 255 when using "unique=True".
  asset.djangocontenttype: "app_label": CharField cannot have a "max_length" greater than 255 when using "unique=True".
  asset.djangocontenttype: "model": CharField cannot have a "max_length" greater than 255 when using "unique=True".
  </code></pre>
  
  <p>The thing is that these tables are auto-generated by django. Plus I just checked in the database (mysql) and the fields are varchar(100).</p>
  
  <p>What's wrong ?</p>