How can one pass model instances directly to a Schema without type errors ?

[GTorreil]

Consider the following snippet :

"""
Type error repro for ninja.
"""

from django.db import models
from django.http import HttpRequest
from ninja import Field, Router, Schema


class CatModel(models.Model):
    """
    The Django model for the cat.
    """

    id = models.IntegerField(primary_key=True)
    name = models.CharField(max_length=255)


class CatDTO(Schema):
    """
    Schema for a cat returned by the API.
    """

    id: int
    name: str


class CatCreateResponseDTO(Schema):
    """
    A response with a list of serialized model instances.
    """

    cat: CatDTO = Field(
        description="The created cat.",
    )
    message: str = Field(description="A message that is also part of the response.")


router = Router()


@router.post(
    "/cats",
    summary="Create a new cat.",
    response={200: CatCreateResponseDTO},
)
def create_cat(_request: HttpRequest):
    # Let's pretend we've created a cat.
    model_instance = CatModel.objects.get(id=1)  # model_instance is of type MyModel
    return 200, CatCreateResponseDTO(cat=model_instance, message="New cat created. 🐈")

This works fine at runtime, but Pyright complains about the model instance being passed to the constructor of CatCreateResponseDTO.

Argument of type "CatModel" cannot be assigned to parameter "cat" of type "CatDTO" in function "__init__"
  "CatModel" is incompatible with "CatDTO"Pylance[reportArgumentType](https://github.com/microsoft/pyright/blob/main/docs/configuration.md#reportArgumentType)

Indeed, cat is expected to be a CatDTO, not a CatModel ; but since Django Ninja automatically serialises and validates the model into the appropriate DTO, it is still valid.

I guess I could do CatDTO.from_orm(cat) but this would validate the data twice, wasting compute resources ?

How are we supposed to pass Django models to Ninja Schemas without type errors ?

Thanks a lot :)

[vitalik]

Hi @GTorreil

well I'm not a big fan of making EVERYTHING strictly typed... but in your case I guess you can do the following:

    cat = CatModel.objects.get(id=1)  # model_instance is of type MyModel
    cat_dto = CatDTO.from_orm(cat)
    return 200, CatCreateResponseDTO(cat=cat_dto, message="New cat created. 🐈")

[GTorreil]

Thanks,

While this works, I think I'll rather kill the warning to avoid double-validation.