I currently have this Django models I want to serialize:
class Result(models.Model):
...
routes = models.ManyToManyField(Route)
...
class Route(models.Model):
...
class Feature(models.Model):
result = models.ForeignKey(Result)
route = models.ForeignKey(Route)
description = models.TextField()
And the DRF serializers looks like:
class ResultSerializer(serializers.ModelSerializer):
...
route = RouteSerializer(many=True, required=False)
...
class Meta:
model = Result
fields = '__all__'
class FeatureField(serializers.CharField):
"""
Accepts text in the writes and looks up the correct feature for the reads.
"""
def get_attribute(self, obj):
# We pass the object instance onto `to_representation`, not just the field attribute.
return obj
def to_representation(self, obj):
try:
search_result = self.root.child.instance
# FIXME: this is the problem.
feature = Feature.objects.get(route=obj.id, search_result=search_result)
feature = feature.description
except Feature.DoesNotExist:
feature = None
return feature
class RouteSerializer(serializers.ModelSerializer):
description = FeatureField(required=False)
class Meta:
model = Route
fields = '__all__'
The problem I mean in the code is that this works when I'm using a ResultSerializer with just one instance, but if I want to serialize several instances in a list view for example, and I pass a queryset to the serializer, DRF applies a ListSerializer on top of it and now the self.root.instance is a list of the records, and I can't access the individual Results that call the nested RouteSerializer so I can't retrieve the correct Feature.
I jumped into DRF code and finally understood what was going on:
If you serialize just one instance with serializer = ResultSerializer(result)
, the serializer.instance
contains only this single, particular result
instance, and the nested serializers and fields can access it without problem using self.root.instance
.
Now, if you serialize several instances, like the default list
action does, what really happens is the following:
serializer = ResultSerializer(queryset, many=True)
is performedmany=True
in the arguments triggers the many_init()
method from BaseSerializer
, and this creates a single ResultSerializer
with the queryset as instance, so serializer.instance
is the queryset.ListSerializer
extending ResultSerializer
and its instance again is the queryset.What I got wrong is thinking that the ListSerializer
would create separated ResultSerializer
s for each element in the queryset.
How I finally solved this is overriding the ResultSerializer.to_representation()
method:
class ResultSerializer(serializers.ModelSerializer):
def to_representation(self, instance):
# When we call Results with many=True, the serializer.instance is a list with several records,
# we can't know which particular instance is spawning the nested serializers so we add it here.
self._instance = instance
return super(ResultSerializer, self).to_representation(instance)
and finally consume it in the FeatureField like this:
class FeatureField(serializers.CharField):
"""
Accepts text in the writes and looks up the correct feature for the reads.
"""
def get_attribute(self, obj):
# We pass the object instance onto `to_representation`, not just the field attribute.
return obj
def to_representation(self, obj):
# If the root is a ListSerializer, retrieve the right Result instance using the `_instance` attribute.
try:
if isinstance(self.root, serializers.ListSerializer):
search_result = self.root.child._instance
else:
search_result = self.root.instance
feature = Feature.objects.get(route=obj.id, search_result=search_result)
feature = feature.pickup_instructions
except Feature.DoesNotExist:
feature = None
return feature