Django: unable to create Generic Foreign key object - django

Question

I'm using Django 2.x
I have two models
class DynamicUrlObject(models.Model):
content_type = models.ForeignKey(ContentType, null=True, on_delete=models.SET_NULL)
object_id = models.PositiveIntegerField()
content_object = GenericForeignKey('content_type', 'object_id')
domain = models.CharField(max_length=255)
and
class MyModel(models.Model):
name = models.Char(max_length=50)
my_obj = fields.GenericRelation(DynamicUrlObject, related_query_name='my_obj')
I have an object of MyModel and want to create a record for DynamicUrlObject and link same to the MyModel model.
I'm doing it something like
dy_obj = DynamicUrlObject.objects.get_or_create(
content_object=my_existing_obj,
domain='http://example.com'
)
my_existing_obj.my_obj = dy_obj
my_existing_obj.save()
But this is not creating a record for DynamicUrlObject and gives an error as
django.core.exceptions.FieldError: Field 'content_object' does not generate an
automatic reverse relation and therefore cannot be used for reverse querying.
If it is a GenericForeignKey, consider adding a GenericRelation.

Answer 1

You cannot filter or get directly on a generic foreign key [1], so get_or_create() won't work. If you know the type of my_existing_obj is MyModel, you can use the GenericRelation you set on MyModel:
try:
dy_obj = DynamicUrlObject.objects.get(my_obj=my_existing_object, domain=...) # use the `related_query_name` here
except DynamicUrlObject.DoesNotExist:
dy_obj = DynamicUrlObject(content_object=my_existing_object, domain=...)
dy_obj.save()
Also once you've created dy_obj, you don't need to assign the reverse relationship to my_existing_object. The GenericRelation isn't a concrete field in the db, it's just a way for django ORM to know how to name the relationships.
[1] https://docs.djangoproject.com/en/2.2/ref/contrib/contenttypes/#django.contrib.contenttypes.fields.GenericForeignKey