Why does this OneToOneField reverse relationship not persist to the database? - django

Question

Relevant frameworks: Django, Django REST Framework
On POST everything seems fine: a 201 Created response is returned and the JSON representation of what should have been persisted to the database in the response body is correct.
In reality, the reverse relationship between Flag and Product has not been established (see relevant models, serializer, and view below) and Product.flag is actually None (serialized to null by DRF).
Relevant models:
class Product(models.Model):
...
flag = models.OneToOneField('Flag', null=True, blank=True, related_name='product')
class Flag(models.Model):
...
reasons = ManyToManyField('FlagReason')
severity = models.CharField(...)
notes = models.TextField(...)
...
Relevant DRF serializer:
class FlagSerializer(serializers.ModelSerializer):
class Meta:
model = models.Flag
fields = ('id', 'reasons', 'notes', 'created_timestamp',
'modified_timestamp', 'severity', 'product',)
Relevant DRF view:
class FlagListCreate(generics.ListCreateAPIView):
queryset = Flag.objects.all()
serializer_class = FlagSerializer
filter_fields = ('severity',)
POST request JSON body:
{
"product": 1328,
"severity": "mid",
"reasons": [1, 2],
"notes": "Test note",
}
201 Created response to the above POST request (note product pk):
{
"id": 31,
"reasons": [
1,
2
],
"notes": "Test note",
"created_timestamp": "2018-04-30T11:54:50.762054+01:00",
"modified_timestamp": "2018-04-30T11:54:50.762084+01:00",
"severity": "mid",
"product": 1328
}
Response to GET request for above Flag (note product value is null):
{
"id": 31,
"reasons": [
1,
2
],
"notes": "Test note",
"created_timestamp": "2018-04-30T11:54:50.762054+01:00",
"modified_timestamp": "2018-04-30T11:54:50.762084+01:00",
"severity": "mid",
"product": null
}