title: added new boolean field to an existing django Mongoengine model, but after can not filter for this field
body: <p>First my django model was like this:</p>
  
  <pre><code>class List(Document):
      owner = ReferenceField('User')
      name = StringField()
      users = ListField(ReferenceField('User'))
      created_at = DateTimeField(default=datetime.datetime.now)
  </code></pre>
  
  <p>After I added a new filed is_cancelled and now it is like that:</p>
  
  <pre><code>class List(Document):
      owner = ReferenceField('User')
      name = StringField()
      users = ListField(ReferenceField('User'))
      created_at = DateTimeField(default=datetime.datetime.now)
      is_cancelled = BooleanField(default = False)
  </code></pre>
  
  <p>I use mongoengine for django mongodb ORM. But now when i want to make a filter query:</p>
  
  <pre><code>List.objects.filter(is_cancelled=False)
  returns []
  </code></pre>
  
  <p>I make all is_cancelled fields to False with django object:</p>
  
  <pre><code>for x in List.objects.all():
      x.is_cancelled = False
      x.save()
  </code></pre>
  
  <p>But I'm still getting an empty list for the query above. 
  I'm looking a django objects' is_cancelled filed and I see is_cancelled = False</p>
  
  <pre><code>l = List.objects.all()[0]
  l.is_cancelled
  </code></pre>
  
  <p>False</p>
  
  <p>But when I look from mongodb shell. There is no filed as is_cancelled. </p>
  
  <pre><code>db.list.find()
  { "_cls" : "List", "_id" : ObjectId("4e8451598ebfa80228000000"), "_types" : [ "List" ],     
  "created_at" : ISODate("2011-09-29T16:24:28.781Z"), "name" : "listname", "users" : [          
  {
              "$ref" : "user",
              "$id" : ObjectId("4e79caf78ebfa80c00000001")
      },      {
              "$ref" : "user",
              "$id" : ObjectId("4e79e4df8ebfa80b64000001")
      },      {
              "$ref" : "user",
              "$id" : ObjectId("4e7aeb898ebfa80b64000001")
      },      {
              "$ref" : "user",
              "$id" : ObjectId("4e79ce028ebfa80c00000004")
      } ] }    
  </code></pre>
  
  <p>How can I fix this query</p>