Use RootModel as query parameter

[Kludex]

Discussed in #11101

^{Originally posted by WarpedPixel February 6, 2024}

First Check

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Example Code

# fail.py
# uvicorn fail:app --reload

from fastapi import FastAPI
from pydantic import RootModel

SimpleID = RootModel[str]

app = FastAPI()

@app.get("/test1")
async def t1(param1: SimpleID):     # BUG: expected to be query param, but it is body
    return {"message": f"Hello {param1}"}

# @app.get("/test2")
# async def t2(param1: Annotated[SimpleID, Query()]): # this workaround does NOT work
#     return {"message": f"Hello {param1}"}

@app.get("/test3")
async def t3(param1: SimpleID, param2: str):  # param2 is query param as expected, param1 is NOT
    return {"message": f"Hello {param1} {param2}"}

Description

Simple types should be usable as query params. Pydantic provides RootModel (instead of BaseModel) precisely to implement types with rich validation based on native types like list or str (instead of the more typical object/dict). Types built with RootModel do not seem to work as query params, even if it is functionally equivalent to a simple type like str.
Furthermore, we cannot force them to work as query params with Query() annotations either.
NOTE: the example is overly simplified, my RootModel-derived types do much more in terms of validation.

Operating System

macOS

Operating System Details

No response

FastAPI Version

0.108.0

Pydantic Version

2.5.3

Python Version

Python 3.11.4

Additional Context

I am not sure this is possible, but I would like for FastAPI to understand RootModel derived types, if the root type is one of the types that can be used as query param, then it should work as such.
The workaround in my case is to use str parameters in my FastAPI calls then manually convert to the rich Pydantic type that does validation. Which kind of defeats the purpose of FastAPI rich type annotations.

[marianhlavac]

Stumbled upon this today, I was quite surprised this is not possible, this feels very FastAPI-like 😄 Is this feature difficult to implement?

I get the feeling that running validations on such query fields can be problematic. That's was btw my primary motivation to do this.

Just to provide another example, this is my use-case I wanted to originally implement. The idea was to have consistent validator for this type, and to have convenience methods on top of this value.

class BoundingBox(RootModel[str]):
    root: str = Field(pattern=BBOX_REGEX)

    def expand_values(self):
        ...

    @root_validator
    def validate_number_ranges(cls, value):
        ...

@router.get("/")
def foo(
    bbox: BoundingBox | None,
):
    exapnded_values = bbox.expand_values()

[MarkusSintonen]

@Kludex, @marianhlavac adding support is easy so I did it here: #11306