Serialization Through Schema and ModelSchema Classes

Hi, I may be missing something very obvious but I’m struggling with nested schema in Django Ninja. Let’s consider 2 models:

class Product(models.Model):
    name = models.CharField(max_length=200)

and

class Category(models.Model):
    product = models.ForeignKey(Product, on_delete=models.CASCADE)
    category = models.CharField(max_length=5)
    expiry_date = models.DateField()

Ninja documentation shows only one nested example, applied to my models, it would look something like that:

class CategorySchema(ModelSchema):
    class Config:
        model = Category
        model_fields = ['category', 'expiry_date]

class ProductSchema(ModelSchema):
    category = CategorySchema = None
    class Config:
        model = Product
        model_fields = ['name']

The issue I have is that it only works if there is one unique category. I tried to change it to category = List[CategorySchema] but it doesn’t work. What am I missing? How to write a Schema that will return list of nested objects within an object?

Hi Tom,

I wish I knew an easy answer for you. I haven’t done this myself. I took a quick look through the docs and couldn’t find an example that matched yours. Ninja is built on top of Pydantic, so I took a look through theirs as well, and it implies it might be able to be done, but the examples provided are single nesting like you’ve seen.

A few things you might try:

1. Add another layer of indirection: have a CategoryListingSchema which is built on a list of Categories.
2. Check the Pydantic documentation in detail to see if their “list” types can solve your problem
3. Post to the Ninja guys posing the question (django-ninja.rest-framework.com/help/). The one time I had a question they were pretty responsive.

Sorry I couldn’t be more help. If you figure it out, post back here in case others have the same question.

Thank you Christopher. I will do some more research.

I have one following question if you don’t mind. Apart from nesting, is there any other obvious way to use multiple schemas in a single response? So, 200 would return both product ProductSchema and CategorySchema regardles if one is nested in the other?

Hi Tom,

Your sample code is using ModelSchemas, which are actually shortcuts so you don’t have to write as much code when your Schema maps to a Django Model nicely. There is also a more basic component called just a Schema (it’s been a while since I wrote this course I can’t recall whether I cover it or not).

With a regular Schema you can build whatever you like, including multiple model references. It just means more work. There is also a mechanism called a resolver function that allows you to write a function in a schema that dictates how a field is populated, so you can do some very fine grained control.

django-ninja.rest-framework.com/guides/response/

I’m a big fan of Ninja, it does the common stuff quickly and well. The Django REST Framework (DRF) is far more comprehensive though, so if you’re finding you’re hitting limitations of what you’re trying to accomplish, the DRF might be a better fit for you. Much more coding there though.

There is a course on the DRF as well, as long as you’re not tired of listening to me talk :)

realpython.com/courses/django-rest-framework/

Chris, I’ve done your DRF first and I believe you directed me to Ninja which seems far better suited to my needs. Thanks for that!

The problem is now solved. Nesting a list of schemas requires just two steps. Correct declaration in Schema is, in my case: category: List[CategorySchema] = [] Second step is in the api function, I had to convert the queryset to a list of dicts with categories = [category.__dict__ for category in categories] before passing it to the response as otherwise I received value is not a valid list (type=type_error.list) error.

Hey Tom,

Glad it worked out. Post a code snippet if you’ve got a minute, would definitely be helpful to others trying to accomplish the same thing. …ct