Nested serializer doesn't pick up correct ID - django-rest-framework

Question

There are two models, they are defined this way:
class ShoppingList(models.Model):
id = models.CharField(max_length=40, primary_key=True)
name = models.CharField(max_length=40)
session_id = models.CharField(max_length=40)
config_file = models.FileField(upload_to=upload_config_file)
def __str__(self):
return self.id
class FetchedData(models.Model):
model_id = models.CharField(max_length=40)
config_id = models.ForeignKey(BillOfMaterial, on_delete=models.CASCADE, default=0)
config_name = models.CharField(max_length=40)
def __str__(self):
return self.model_id
And serialized like this:
class FetchedDataSerializer(serializers.ModelSerializer):
file_fields = serializers.SerializerMethodField()
class Meta:
model = FetchedData
fields = ('model_id', 'config_id', 'config_name', 'file_fields')
def get_file_fields(self, obj):
print(obj)
# queryset = ShoppingList.objects.filter(config_file = obj) ## (1)
queryset = BillOfMaterial.objects.all() ## (2)
return [ShoppingListSerializer(cf).data for cf in queryset]
I was advised* to implement the solution marked as (1) in the serializer above, but when it's on, I get responses with an empty array, for example:
[
{
"model_id": "6553",
"config_id": "2322",
"config_name": "Config No. 1",
"file_fields": []
}
]
Meanwhile, with option (2) turned on and option (1) commented out, I get all the instances displayed:
[
{
"model_id": "6553",
"config_id": "2322",
"config_name": "Config No. 1",
"file_fields": [
{
"id": "2322",
"name": "First Example",
"session_id": "9883",
"config_file": "/uploads/2322/eq-example_7DQDsJ4.json"
},
{
"id": "4544",
"name": "Another Example",
"session_id": "4376",
"config_file": "/uploads/4544/d-jay12.json"
}
]
}
]
The print(obj) method always gives a model_id value. And it should output file_fields.id, I guess.
How should I re-build this piece of code to be able to display only the file_field with id matching config_id of the parent?
*This is a follow-up of an issue described here: TypeError: 'FieldFile' object is not callable

Answer 1

In FetchedData model I added this method:
def config_link(self):
return self.config_id.config_file
(it binds config_file from ShoppingList model).
FetchedDataSerializer should then look like this:
class FetchedDataSerializer(serializers.ModelSerializer):
file_link = serializers.SerializerMethodField()
class Meta:
model = FetchedData
fields = ('model_id', 'config_id', 'config_name', 'file_link')
def get_file_link(self, obj):
return obj.config_link()