EmbeddedModelField serialization error

[myaser]

I have two models Index is referencing Posting as follows:

class Posting(models.Model):

    document = models.ForeignKey(Tweet)
    positions = mongo_fields.ListField()


class Index(models.Model):

    token = models.CharField(max_length=10, unique=True)
    postings = mongo_fields.ListField(mongo_fields.EmbeddedModelField('Posting'))
    term_frequency = models.IntegerField()

when I serialize Index objects I get an error

Traceback (most recent call last):
  File "<input>", line 1, in <module>
    serializers.serialize('json', Index.objects.all())
  File ".../django/core/serializers/__init__.py", line 91, in serialize
    s.serialize(queryset, **options)
  File ".../django/core/serializers/base.py", line 54, in serialize
    self.end_serialization()
  File ".../django/core/serializers/json.py", line 21, in end_serialization
    simplejson.dump(self.objects, self.stream, cls=DjangoJSONEncoder, **self.options)
  File ".../simplejson/__init__.py", line 261, in dump
    for chunk in iterable:
  File ".../simplejson/encoder.py", line 600, in _iterencode
    for chunk in _iterencode_list(o, _current_indent_level):
  File ".../simplejson/encoder.py", line 455, in _iterencode_list
    for chunk in chunks:
  File ".../simplejson/encoder.py", line 568, in _iterencode_dict
    for chunk in chunks:
  File ".../simplejson/encoder.py", line 568, in _iterencode_dict
    for chunk in chunks:
  File ".../simplejson/encoder.py", line 455, in _iterencode_list
    for chunk in chunks:
  File ".../simplejson/encoder.py", line 622, in _iterencode
    o = _default(o)
  File ".../django/core/serializers/json.py", line 58, in default
    return super(DjangoJSONEncoder, self).default(o)
  File ".../simplejson/encoder.py", line 239, in default
    raise TypeError(repr(o) + " is not JSON serializable")
TypeError: <Posting: {'positions': [12], 'document': u'5218d561c454fe2ae837019c', 'id': None}> is not JSON serializable

while Posting objects is serializable

x = Posting.objects.create(document=tweet, positions=[1,2,3])
serializers.serialize('json', [x])
'[{"pk": "52194e0ec454fe20bbcb841d", "model": "indexer.posting", "fields": {"positions": [1, 2, 3], "document": "5218d561c454fe2ae83701a4"}}]'

I think the problem is that Posting objects which are embedded in Index have id == None