RedisOM Pydantic objects cause POST request errors with FastAPI

[aleexharris]

If using a Redis-OM Pydantic object as the input for a FastAPI route with a version greater than 0.112, you get the error shown below. NOTE: You don't even need to be trying to access the underlying RedisDB for this issue to occur, it is native to the Redis-OM wrapper around Pydantic.

To recreate for yourself:

# pyproject.toml

dependencies = [
    "fastapi[standard]==0.113",
    "redis-om>=0.3.3",
]

# main.py

from fastapi import FastAPI
from redis_om import JsonModel

class Name(JsonModel):
    greet: str

app = FastAPI()

@app.post("/greet")
async def greet(name: Name):
    # NOTE: You don't even need to interact with name to cause an error
    return {"message": "Hello World"}

The error:

pydantic.errors.PydanticSchemaGenerationError: Unable to generate pydantic-core schema for <class 'redis_om.model.model.ExpressionProxy'>. Set arbitrary_types_allowed=True in the model_config to ignore this error or implement __get_pydantic_core_schema__ on your type to fully support it.
If you got this error by calling handler() within __get_pydantic_core_schema__ then you likely need to call handler.generate_schema(<some type>) since we do not call __get_pydantic_core_schema__ on <some type> otherwise to avoid infinite recursion.
For further information visit https://errors.pydantic.dev/2.10/u/schema-for-unknown-type

[AdolfoVillalobos]

Hi! Im experiencing the same issue. Did you find a solution?

[aleexharris]

Hi! Im experiencing the same issue. Did you find a solution?

The only solution I have found is to cap the version of my FastAPI import to 0.112 and then work everything around that.

# pyproject.toml
dependencies = [
    "fastapi[standard]==0.112",
    "redis-om>=0.3.3",
]

I believe this means that under-the-hood, you end up using Pydantic version 2.8.2 via RedisOM.

[abrookins]

Testing Results - Issue Appears Fixed

I tested this issue with the current main branch and FastAPI 0.128.0. The issue no longer reproduces.

Test Code

from contextlib import asynccontextmanager
from fastapi import FastAPI
from fastapi.testclient import TestClient
from aredis_om import HashModel, Field, Migrator

class SimpleModel(HashModel, index=True):
    name: str = Field(index=True)
    value: int = Field(index=True)

@asynccontextmanager
async def lifespan(app: FastAPI):
    await Migrator().run()
    yield

app = FastAPI(lifespan=lifespan)

@app.post("/test")
async def test_direct(model: SimpleModel):
    return {"pk": model.pk}

# Test it
from httpx import ASGITransport
import httpx, asyncio
from asgi_lifespan import LifespanManager

async def test():
    async with LifespanManager(app):
        transport = ASGITransport(app=app)
        async with httpx.AsyncClient(transport=transport, base_url="http://test") as client:
            r = await client.post("/test", json={"name": "test", "value": 42})
            print(r.status_code, r.json())

asyncio.run(test())
# Output: 200 {'pk': '01KFM1HH3RHY8Q88D1TSJEQ0MY'}

Result

✅ Request succeeded - No PydanticSchemaGenerationError for ExpressionProxy

Environment

• redis-om: current main branch (post-1.0)
• FastAPI: 0.128.0
• Pydantic: 2.x
• Python: 3.12

This may have been fixed as part of the Pydantic v2 migration or other recent changes. Reopen if you still see this in main!