title: How to save a list of blob keys that belong to an entity?
body: <p>With my webapp users can upload images as blobs in GAE. I want an easy connection between the article and the images (blobs) so I tried the following:</p>
  
  <pre><code>class Article(db.Model):
      blobs = db.ListProperty(db.Key)
  </code></pre>
  
  <p>and then in the blobstoreuploadhandler:</p>
  
  <pre><code>for upload in self.get_uploads():
      try:
          article.blobs.append(upload.key())
  </code></pre>
  
  <p>But then it gets an exception:</p>
  
  <pre><code> /upload_form 404 3753ms 1kb Mozilla/5.0 (Windows NT 6.1; rv:28.0) Gecko/20100101 Firefox/28.0 module=default version=leakagefix
  0.1.0.30 - - [20/Mar/2014:04:46:27 -0700] "POST /upload_form HTTP/1.1" 404 1331 "http://www.koolbusiness.com/ai" "Mozilla/5.0 (Windows NT 6.1; rv:28.0) Gecko/20100101 Firefox/28.0" "www.koolbusiness.com" ms=3754 cpu_ms=2036 cpm_usd=0.000149 app_engine_release=1.9.1 instance=00c61b117c8167bb98b5f7f69bd68254df45fe64
  E 2014-03-20 12:46:27.190
  There was an exception:Items in the blobs list must all be Key instances
  E 2014-03-20 12:46:27.216
  Items in the blobs list must all be Key instances
  </code></pre>
  
  <p>Can you tell me how it should be done and what was wrong with this attempt?</p>