Django create POST intermediate model - django

Question

I'm using DRM v3.9.4 and have the following models:
class Book(models.Model):
id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
...
users = models.ManyToManyField("User", related_name="books", through="BookUser")
class BookUser(models.Model):
user = models.ForeignKey("User", on_delete=models.CASCADE)
book = models.ForeignKey("Book", on_delete=models.CASCADE)
permission = models.CharField(...)
class User(models.Model):
id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
...
with the following serializers:
class UserSerializer(ModelSerializer):
class Meta:
model = User
fields = ( "id", ... )
class BookUserSerializer(ModelSerializer):
class Meta:
model = BookUser
fields = ('user', 'permission')
class BookSerializer(ModelSerializer):
users = BookUserSerializer(source='bookuser_set', many=True)
class Meta:
model = Book
fields = ( "id", "users", ... )
extra_kwargs = {
"users": {"allow_empty": True},
}
When reading (GET) a Book everything is fine:
{
id: ...,
users: [
{
user: 'some-uuid',
permission: 'read-only'
},
...
]
}
but when trying to POST using the same payload:
{
id: ...,
users: [
{
user: 'some-uuid',
permission: 'read-only'
},
...
]
}
I get an error:
KeyError: 'users'
looking at the api-guide (https://www.django-rest-framework.org/api-guide/relations/)
seems that by default nested serializers are read-only,
but I can't make it work.
BTW, IMPORTANT: all the users are (and should) already exist, so I expect this POST call to add a record in the intermediate BookUser table, and ofcourse the Book itself, but NOT to actually add new User(s) records.
Any help, guiding or assistance would be appreciate as how to make this thing work

Answer 1

Using drf-writable-nested package you can achieve that:
from drf_writable_nested import WritableNestedModelSerializer
class BookSerializer(WritableNestedModelSerializer):
users = BookUserSerializer(source='bookuser_set', many=True)
class Meta:
model = Book
fields = ( "id", "users", ... )
extra_kwargs = {
"users": {"allow_empty": True},
}
DRF Writable nested