Django Rest Framework JSON API - Update M2M relationship not persisting - django

Question

I'm trying to update a resource in a PATCH.
The resource is updated fine, but the resources in the M2M table don't change.
Models
class StatementOfAdvice(Model):
id = HashidUnsignedAutoField(primary_key=True, salt="StatementOfAdvice", min_length=15)
loan_purposes = ManyToManyField(
to=StaticLoanPurpose, through=StatementOfAdviceLoanPurpose, related_name="loan_purposes"
)
class StaticLoanPurpose(Model):
id = CharField(db_column="loan_purpose_id", primary_key=True, max_length=150)
value = CharField(max_length=150, unique=True)
class StatementOfAdviceLoanPurpose(Model):
id = HashidUnsignedAutoField(primary_key=True, salt="StatementOfAdviceLoanPurpose", min_length=15)
statement_of_advice = ForeignKey(to="StatementOfAdvice", on_delete=DO_NOTHING)
loan_purpose = ForeignKey(to="StaticLoanPurpose", on_delete=DO_NOTHING)
Serializers
class StatementOfAdviceSerializer(Serializer):
included_serializers = {"client_account": ClientAccountSerializer, "loan_purpose": StaticLoanPurposeSerializer}
loan_purposes = ResourceRelatedField(many=True, read_only=False, queryset=StaticLoanPurpose)
class Meta:
model = StatementOfAdvice
fields = "_all_"
class StaticLoanPurposeSerializer(Serializer):
class Meta:
model = StaticLoanPurpose
fields = "_all_"
My PATCH request: http://localhost:8000/statements_of_advice/zELX1KdyZjgQGkp/
Payload:
{
"data": {
"type": "StatementOfAdvice",
"attributes": {},
"relationships": {
"loan_purposes": {
"data": [
{
"type": "StaticLoanPurpose",
"id": "construct_io"
},
{
"type": "StaticLoanPurpose",
"id": "other_purpose"
},
{
"type": "StaticLoanPurpose",
"id": "purchase_io"
}
]
}
},
"id": "zELX1KdyZjgQGkp"
}
}
The result I expect from this PATCH request is 3 records in the linking table StatementOfAdviceLoanPurpose. But I get none.
If anyone could help me here I would greatly appreciate it.

Answer 1

Ok, Then. The solution I found was actually using rest_framework_json_api.views.AutoPrefetchMixin in my StatementOfAdviceViewSet.
class StatementOfAdviceViewSet(BaseViewSet, AutoPrefetchMixin):
queryset = StatementOfAdvice.objects.all()
serializer_class = StatementOfAdviceSerializer
filterset_class = StatementOfAdviceFilterSet
Reference link:
https://django-rest-framework-json-api.readthedocs.io/en/stable/apidoc/rest_framework_json_api.views.html?highlight=manytomany#rest_framework_json_api.views.AutoPrefetchMixin.get_queryset
But this only fixes part of the problem. Now I can insert records in the linking table using the same payload as in the question.
But I cannot remove them. I tried changing the request method to PUT assuming that since PUT is supposed to replace all values of the resource the framework would handle it to me. But it doesn't.
So besides adding the AutoPrefetchMixin in the viewset. I also had override the update() function in the StatementOfAdviceSerializer to achieve the desired behaviour.
The advantage of using AutoPrefetchMixin is that you don't have to override the create() method.