enable MISSING and NoDefault sentinel

[Ksauder]

#1655

I've been struggling with model schemas having null as an option for fields that I simply want to omit from the response. If the keys don't exist in the response, then there is no data. The current implementation does not allow changing how create_schema handles nullable fields and it always assigns a type | None = None. This causes all of my generated api clients to type hint incorrectly which is really annoying. exclude_unset also does not work for modelschemas since it's technically set since the model instance is interrogated via attributes and returns an explicit None for nullable fields and it does not help generate an accurate json schema.

The main feature of this PR is allowing modification to the type and value used when a field is "nullable". This allows MISSING to be used, which will create the data schema I'm looking for, while also supporting other type/value combos.

from typing import Literal

from django.db import models
from ninja.orm import create_schema
from ninja import NinjaAPI
from pydantic.experimental.missing_sentinel import MISSING

from devtools import debug


class Profile(models.Model):
    name = models.CharField(max_length=10)
    nullable = models.CharField(max_length=10, blank=True, null=True)

    class Meta:
        app_label = "tests"


ProfileModelSchema1 = create_schema(
    Profile,
    name="ProfileModelSchema1",
    fields=["name", "nullable"],
    nullable_type=MISSING,
    nullable_value=MISSING,
)


ProfileModelSchema2 = create_schema(
    Profile,
    name="ProfileModelSchema2",
    fields=["name", "nullable"],
    nullable_type=Literal["MyNullValue"],
    nullable_value="MyNullValue",
)

api = NinjaAPI()


@api.get("/profile1", response=ProfileModelSchema1)
def get_profile1(request):
    return Profile(name="Name")


@api.get("/profile2", response=ProfileModelSchema2)
def get_profile2(request):
    return Profile(name="Name")

api.get_openapi_json() == {
        ...
        'components': {
            'schemas': {
                'ProfileModelSchema1': {
                    'properties': {
                        'name': {
                            'maxLength': 10,
                            'title': 'Name',
                            'type': 'string',
                        },
                        'nullable': {
                            'maxLength': 10,
                            'title': 'Nullable',
                            'type': 'string',
                        },
                    },
                    'required': ['name'],
                    'title': 'ProfileModelSchema1',
                    'type': 'object',
                },
                'ProfileModelSchema2': {
                    'properties': {
                        'name': {
                            'maxLength': 10,
                            'title': 'Name',
                            'type': 'string',
                        },
                        'nullable': {
                            'anyOf': [
                                {
                                    'maxLength': 10,
                                    'type': 'string',
                                },
                                {
                                    'const': 'MyNullValue',
                                    'type': 'string',
                                },
                            ],
                            'default': 'MyNullValue',
                            'title': 'Nullable',
                        },
                    },
                    'required': ['name'],
                    'title': 'ProfileModelSchema2',
                    'type': 'object',
                },
            },
        },
        ...
    }

[vitalik]

hi @Ksauder - would you be able to add some matix to test with pydantic < 2.11 - would not run against all python versions but just one latest

[Ksauder]

Sure I can look into that. I'm still working out actually using this in practice. I'm debugging some issues when a schema with MISSING is actually used as a return schema from an operation.

[Ksauder]

@vitalik Okay, I think this is a pretty clean implementation. Added two args to create_schema and relevant child functions nullable_value and nullable_type with default from conf, NULLABLE_FIELD_*. Had to do this to get MISSING down to the model schema level.
And added MISSING to the check in default_schema so we don't try to encode it.

[Ksauder]

hi @Ksauder - would you be able to add some matix to test with pydantic < 2.11 - would not run against all python versions but just one latest

@vitalik Actually, I'm not sure how you would like this implemented. I'm not familiar with the github CICD, and I assume you want it there. How exactly would you like this done?

[Ksauder]

@vitalik Hi, I just rebased, cleaned up, and got the test matrix done. Any interest in this or anything I can do to move it forward?