How to use Arrow type in FastAPI response schema?

Question

I want to use Arrow type in FastAPI response because I am using it already in SQLAlchemy model (thanks to sqlalchemy_utils).

I prepared a small self-contained example with a minimal FastAPI app. I expect that this app return product1 data from database.

Unfortunately the code below gives exception:

Exception has occurred: FastAPIError
Invalid args for response field! Hint: check that <class 'arrow.arrow.Arrow'> is a valid pydantic field type

import sqlalchemy
import uvicorn
from arrow import Arrow
from fastapi import FastAPI
from pydantic import BaseModel
from sqlalchemy import Column, Integer, Text, func
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker
from sqlalchemy_utils import ArrowType

app = FastAPI()

engine = sqlalchemy.create_engine('sqlite:///db.db')
Base = declarative_base()

class Product(Base):
    __tablename__ = "product"
    id = Column(Integer, primary_key=True, autoincrement=True)
    name = Column(Text, nullable=True)
    created_at = Column(ArrowType(timezone=True), nullable=False, server_default=func.now())

Base.metadata.create_all(engine)


Session = sessionmaker(bind=engine)
session = Session()

product1 = Product(name="ice cream")
product2 = Product(name="donut")
product3 = Product(name="apple pie")

session.add_all([product1, product2, product3])
session.commit()


class ProductResponse(BaseModel):
    id: int
    name: str
    created_at: Arrow

    class Config:
        orm_mode = True
        arbitrary_types_allowed = True


@app.get('/', response_model=ProductResponse)
async def return_product():

    product = session.query(Product).filter(Product.id == 1).first()

    return product

if __name__ == "__main__":
    uvicorn.run(app, host="localhost", port=8000)

requirements.txt:

sqlalchemy==1.4.23
sqlalchemy_utils==0.37.8
arrow==1.1.1
fastapi==0.68.1
uvicorn==0.15.0

This error is already discussed in those FastAPI issues:

1. https://github.com/tiangolo/fastapi/issues/1186
2. https://github.com/tiangolo/fastapi/issues/2382

---

One possible workaround is to add this code (source):

from pydantic import BaseConfig
BaseConfig.arbitrary_types_allowed = True

It is enough to put it just above @app.get('/'..., but it can be put even before app = FastAPI()

The problem with this solution is that output of GET endpoint will be:

// 20210826001330
// http://localhost:8000/

{
  "id": 1,
  "name": "ice cream",
  "created_at": {
    "_datetime": "2021-08-25T21:38:01+00:00"
  }
}

instead of desired:

// 20210826001330
// http://localhost:8000/

{
  "id": 1,
  "name": "ice cream",
  "created_at": "2021-08-25T21:38:01+00:00"
}

Answer 1

Add a custom function with the @validator decorator that returns the desired _datetime of the object:

class ProductResponse(BaseModel):
    id: int
    name: str
    created_at: Arrow

    class Config:
        orm_mode = True
        arbitrary_types_allowed = True

    @validator("created_at")
    def format_datetime(cls, value):
        return value._datetime

Tested on local, seems to be working:

$ curl -s localhost:8000 | jq
{
  "id": 1,
  "name": "ice cream",
  "created_at": "2021-12-02T08:25:10+00:00"
}