Pydantic v2 with v1 compatibility

[roflatelie]

Problem

When using fastapi-pagination with Pydantic V1 models in response_model, a validation error occurs during response serialization.

Versions

• Python: 3.9.10
• FastAPI: 0.119.0
• fastapi-pagination: 0.9.1
• Pydantic: 2.4.2 (but models are created using pydantic.v1)

Details

I am using Page[PydanticV1Model] in the response_model of a FastAPI endpoint. The models are defined via pydantic.v1.BaseModel, while Page from fastapi-pagination uses the Pydantic V2 API. When validating the response, a conflict arises between V1 and V2.

Example

from fastapi import FastAPI
from fastapi_pagination import Page, create_page
from fastapi_pagination.default import Params
from pydantic.v1 import BaseModel

app = FastAPI()

class MyModel(BaseModel):  # Pydantic V1 model
    id: int
    name: str

@app.get("/items", response_model=Page[MyModel])
async def get_items(page_params: Params = Depends()):
    items = [MyModel(id=1, name="test")]
    return create_page(items, total=1, params=page_params)

Error

TypeError: validate() takes 2 positional arguments but 3 were given

Traceback (most recent call last):
  File ".../fastapi/routing.py", line 242, in serialize_response
    value, errors_ = field.validate(response_content, {}, loc=("response",))
  File ".../fastapi/_compat/v2.py", line 115, in validate
    self._type_adapter.validate_python(value, from_attributes=True),
  File ".../pydantic/type_adapter.py", line 441, in validate_python
    return self.validator.validate_python(
  File ".../pydantic/_internal/_validators.py", line 53, in sequence_validator
    v_list = validator(input_value)
TypeError: validate() takes 2 positional arguments but 3 were given

I also get a warning:

UserWarning: Mixing V1 models and V2 models (or constructs, like `TypeAdapter`) is not supported. 
Please upgrade `MyModel` to V2.

Questions

1. Which version of fastapi-pagination officially supports Pydantic V1 models?
2. Is there a way to use Page with Pydantic V1 models without migrating to V2?

[uriyyo]

Hi @roflatelie ,

It's an interesting use case, I will take a look 👀

[uriyyo]

@roflatelie Update from my side.

After a quick look, there is no easy way to do it now. You can either work only with pydantic v2 models, or with pydantic v1 with pydanic<2 installed

[uriyyo]

I will try to come up with smth better, but it might be tricky

[uriyyo]

1. Which version of fastapi-pagination officially supports Pydantic V1 models?

@roflatelie Latest version supports Pydantic V1 models, but you need to have pydantic v1 installed on your system, not pydantic v2 with BaseModel import from pydantic.v1.

pip install 'pydantic<2'

[uriyyo]

Hi @roflatelie,

This issue should be fixed in new version 0.15.1.

Here is example on how to use pydantic v1:

from fastapi import FastAPI
from fastapi_pagination import add_pagination, Page, paginate
from fastapi_pagination.customization import CustomizedPage, UsePydanticV1
from pydantic.v1 import BaseModel

app = FastAPI()
add_pagination(app)

PageV1 = CustomizedPage[
    Page,
    UsePydanticV1(),
]


class UserOut(BaseModel):
    id: int
    name: str


USERS = [
    {"id": 0, "name": "John"},
    {"id": 1, "name": "Jane"},
    {"id": 2, "name": "Doe"},
]


@app.get("/users")
def get_users() -> PageV1[UserOut]:
    return paginate(USERS, safe=True)


if __name__ == "__main__":
    import uvicorn

    uvicorn.run(app)

[uriyyo]

I'm closing this issue, please reopen it if issue is still present on your side 🙏