[BUG] Path and Query parameter Annotated description + title are removed from OpenAPI docs

[jceipek]

Describe the bug Given:

@api.get("/items/{item_id}")
def read_item(request, item_id: Annotated[str, Field(examples=["an example"], title="Some Title", description="A description")]):
    return {"item_id": item_id}

I expect "Some Title" and "A description" show up in the generated OpenAPI docs. However, only "an example" shows up in the generated OpenAPI docs.

Versions (please complete the following information):

• Python version: 3.11.9
• Django version: 5.0.6
• Django-Ninja version: 1.2.0
• Pydantic version: 2.7.4

Root Cause

ninja's class Param(FieldInfo) in ninja.params.models calls:

        super().__init__(
            default=default,
            alias=alias,
            title=title,
            description=description,
            gt=gt,
            ge=ge,
            lt=lt,
            le=le,
            min_length=min_length,
            max_length=max_length,
            json_schema_extra=json_schema_extra,
            **extra,
        )

This ultimately causes a Path instance to be created with these _attributes_set values:

{'default': Ellipsis, 'alias': None, 'title': None, 'description': None, 'gt': None, 'ge': None, 'lt': None, 'le': None, 'min_length': None, 'max_length': None, 'json_schema_extra': {}}

Note that the values of title and description are None.

Direct Cause

When ninja constructs an Operation, it creates a ViewSignature, which calls _create_models. The end of the _create_models function calls model_cls = type(cls_name, (base_cls,), attrs). This causes Pydantic's ModelMetaclass __new__ method to be called, which results in calls to set_model_fields, collect_model_fields, and from_annotated_attribute. from_annotated_attribute calls merge_field_infos on a tuple of three elements (I'm not sure why the first two elements are identical):

FieldInfo(annotation=NoneType, required=True, title='Some Title', description='A description', examples=['an example'])
FieldInfo(annotation=NoneType, required=True, title='Some Title', description='A description', examples=['an example'])
Path(annotation=str, required=True, json_schema_extra={}, metadata=[FieldInfo(annotation=NoneType, required=True, title='Some Title', description='A description', examples=['an example'])])

The Path element in the tuple has an _attributes_set attribute containing this dictionary:

{'default': Ellipsis, 'alias': None, 'title': None, 'description': None, 'gt': None, 'ge': None, 'lt': None, 'le': None, 'min_length': None, 'max_length': None, 'json_schema_extra': {}}

Note that title and description are set to None, so merge_field_infos overwrites the desired description and title with None.

Potential Solution

This bug disappears if FieldInfo.__init__ in class Param(FieldInfo) does not receive None values in the default case. For example, adding this to the bottom of __init__ in class Param(FieldInfo) addresses the immediate problem:

    initializer: dict[str, Any] = {}
    if alias is not None:
        initializer["alias"] = alias
    if title is not None:
        initializer["title"] = title
    if description is not None:
        initializer["description"] = description
    if gt is not None:
        initializer["gt"] = gt
    if ge is not None:
        initializer["ge"] = ge
    if lt is not None:
        initializer["lt"] = lt
    if le is not None:
        initializer["le"] = le
    if min_length is not None:
        initializer["min_length"] = min_length
    if max_length is not None:
        initializer["max_length"] = max_length

    FieldInfo.__init__(
        self,
        default=default,
        json_schema_extra=json_schema_extra,
        **initializer,
        **extra,
    )

I'd be happy to make a pull request making this change, but first I'd like to know if this is an appropriate solution. Is there an easier way to do this?