DRF: to_representation() called twice on serializer update() - django

Question

My site has a Vue frontend with a DRF backend. I have a model with a multi-value field that looks like this:
stages = StageSerializer(
many=True,
read_only=True
)
In the to_representation() method, I process the stages data into three different elements to facilitate features for the frontend.
def to_representation(self, instance):
results = super().to_representation(instance)
return render_stages(results)
The problem is, when I post (PATCH actually) to an update view, which ends up calling the update() method of the serializer, the to_representation() method seems to be called twice. I could post the code from render_stages(), but I don't see how that could be causing the problem. Suffice to say that it takes this, from the call to super().to_representation():
{
.
. other fields
.
"stages": [
{
"id": 1,
"name": "To Do"
},
{
"id": 2,
"name": "In Progress"
},
{
"id": 3,
"name": "Done"
}
]
}
...and turns it into this:
{
"stages": [
"To Do",
"In Progress",
"Done"
],
"stages_order": "1,2,3",
"stages_data": [
{
"id": 1,
"name": "To Do",
"order": 1
},
{
"id": 2,
"name": "In Progress",
"order": 2
},
{
"id": 3,
"name": "Done",
"order": 3
}
]
}
As you can see, the format of the "stages" element changes after the first pass through to_representation() from a list of dicts to a list of strings. So when to_representation() gets called a second time, it raises an exception.
On a list view, it only gets called once. It's just on an update view that it gets called twice. Can someone explain why that's happening?
Update: More details:
The view. The settings model only has one row - pk=1 - so this is a departure from the norm.
class SettingsUpdate(ViewMetaMixin, UpdateAPIView):
serializer_class = SettingsSaveSerializer
permission_classes = (IsAuthenticated, CanAccessEndpoint)
queryset = Settings.objects.none()
def get_object(self):
obj = Settings.objects.get(pk=1)
self.check_object_permissions(self.request, obj)
return obj
def put(self, request, *args, **kwargs):
return self.patch(request, *args, **kwargs)
def patch(self, request, *args, **kwargs):
response = self.partial_update(request, *args, **kwargs)
return Response({
'results': response.data,
'messages': [
{
'message': _('Settings updated.'),
'message_type': 'status',
}
]
})
The serializer:
class SettingsSaveSerializer(SettingsSerializer):
stages = StageSerializer(
many=True,
read_only=True
)
class Meta:
model = Settings
fields = '__all__'
def to_representation(self, instance):
results = super().to_representation(instance)
return render_stages(results)
...validations omitted...
...create method omitted...
def update(self, instance, validated_data):
stages = get_initial_data_field(self.initial_data, 'stages', many=True)
if stages is not None: # Only process "stages" if it's present in form data
validated_data['stages_order'] = Stage.objects.save_stages(instance, stages)
return super().update(instance, validated_data)