Столбец Django throws не может быть нулевой ошибкой при отправке POST в REST Framework

django.db.utils.IntegrityError: (1048, "Column 'owner_id' cannot be null")
[04/Apr/2016 18:40:58] "POST /api/items/ HTTP/1.1" 500 226814

    let body = JSON.stringify({ url: 'fred', item_type: 'P', owner_id: 2 });

    let headers = new Headers();
    headers.append('Content-Type', 'application/json');

    this.http.post('http://127.0.0.1:8000/api/items/',
        body, {
        headers: headers
      })
      .subscribe(
        data => {
          alert(JSON.stringify(data));
        },
        err => alert('POST ERROR: '+err.json().message),
        () => alert('POST Complete')
      );

class ItemViewSet(viewsets.ModelViewSet):
    queryset = Item.objects.all().order_by('-date_added')
    serializer_class = ItemSerializer

    """
        Use the API call query params to determing what to return

        API params can be:

        ?user=<users_id>&num=<num_of_items_to_return>&from=<user_id_of_items_to_show>
    """

    def get_queryset(self):
        this_user = self.request.query_params.get('user', None)
        restrict_to_items_from_user_id = self.request.query_params.get('from', None)
        quantity = self.request.query_params.get('num', 20)

        if restrict_to_items_from_user_id is not None:
            queryset = Item.objects.filter(owner=restrict_to_items_from_user_id, active=True).order_by('-date_added')[0:int(quantity)]
        elif this_user is not None:
            queryset = Item.objects.filter(active=True, credits_left__gt=0).exclude(pk__in=Seen.objects.filter(user_id=this_user).values_list('item_id', flat=True))[0:int(quantity)]
        else:
            queryset = Item.objects.filter(active=True, credits_left__gt=0)[0:int(quantity)]

        print("User id param is %s  and quantity is %s" % (user_id,quantity))

        return queryset

class Item(models.Model):

    ITEM_TYPES = (
        ('V', 'Vine'),
        ('Y', 'YouTube'),
        ('P', 'Photo'),         # Photo is stored by us on a CDN somewhere
        ('F', 'Flickr'),
        ('I', 'Instagram'),
        ('D', 'DeviantArt'),
        ('5', '500px'),
    )
    owner           = models.ForeignKey(User, on_delete=models.CASCADE)     # Id of user who owns the item
    title           = models.CharField(max_length=60, default='')           # URL of where item resides (e.g. Vine or YouTube url)
    url             = models.CharField(max_length=250, default='')          # URL of where item resides (e.g. Vine or YouTube url)
    item_type       = models.CharField(max_length=1, choices=ITEM_TYPES)    # Type of item (e.g. Vine|YoutTube|Instagram|etc.)
    keywords        = models.ManyToManyField(Keyword, related_name='keywords')
                                                                            # E.g. Art, Travel, Food, etc.
    credits_applied = models.IntegerField(default=10, help_text='Total number of credits applied to this item including any given by VeeU admin')
                                                                            # Records the total number of credits applied to the Item
    credits_left    = models.IntegerField(default=10, help_text='The number of credits still remaining to show the item')
                                                                            # Number of credits left (goes down each time item is viewed
    credits_gifted  = models.IntegerField(default=0, help_text='The number of credits this item has been gifted by other users')
                                                                            # Number of credits users have gifted to this item
    date_added      = models.DateTimeField(auto_now_add=True)               # When item was added
    liked           = models.IntegerField(default=0)                        # Number of times this item has been liked
    disliked        = models.IntegerField(default=0)                        # Number of times this item has been disliked
    active          = models.BooleanField(default=True, help_text='If you mark this item inactive please say why in the comment field. E.g. "Inapproriate content"')
                                                                            # True if item is available for showing
    comment         = models.CharField(max_length=100, blank=True)          # Comment to be applied if item is inactive to say why

    # Add defs here for model related functions

    # This to allow url to be a clickable link
    def item_url(self):
        return u'<a href="%s">%s</a>' % (self.url, self.url)
    item_url.allow_tags = True

    def __str__(self):
        return '%s: Title: %s, URL: %s' % (self.owner, self.title, self.url)

class ItemSerializer(serializers.HyperlinkedModelSerializer):
    username = serializers.SerializerMethodField()

    def get_username(self, obj):
        value = str(obj.owner)
        return value

    def get_keywords(self, obj):
        value = str(obj.keywords)
        return value

    class Meta:
        model = Item
        fields = ('url', 'item_type', 'title', 'credits_applied', 'credits_left', 'credits_gifted', 'username', 'liked', 'disliked')