How to Use Config Class in Pydantic with FastAPI

In FastAPI, use the Config class inside a Pydantic model to customize behavior like enabling orm_mode for ORM compatibility or setting JSON encoders. Define a nested Config class with attributes inside your model to control validation and serialization.

Syntax

The Config class is a nested class inside a Pydantic model. It lets you set options that change how the model behaves during validation and serialization.

• orm_mode: Allows the model to read data from ORM objects.
• json_encoders: Customize how certain types are converted to JSON.
• any_other_option: Other Pydantic settings like allow_population_by_field_name.

python

from pydantic import BaseModel

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

    class Config:
        orm_mode = True
        json_encoders = {
            # custom encoders here
        }

Example

This example shows a Pydantic model with Config enabling orm_mode. It allows FastAPI to return ORM objects directly as JSON responses.

python

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

# Simulate an ORM object
class UserORM:
    def __init__(self, id: int, username: str):
        self.id = id
        self.username = username

class User(BaseModel):
    id: int
    username: str

    class Config:
        orm_mode = True

@app.get("/user", response_model=User)
async def get_user():
    user_orm = UserORM(id=1, username="alice")
    return user_orm

Output

{"id":1,"username":"alice"}

Common Pitfalls

One common mistake is forgetting to set orm_mode = True when returning ORM objects. Without it, FastAPI cannot convert ORM instances to JSON properly and raises errors.

Another pitfall is misconfiguring json_encoders which can cause serialization failures for custom types.

python

from pydantic import BaseModel

# Wrong: missing orm_mode
class UserWrong(BaseModel):
    id: int
    username: str

# Right: orm_mode enabled
class UserRight(BaseModel):
    id: int
    username: str

    class Config:
        orm_mode = True

Quick Reference

Config Option	Purpose	Example Value
orm_mode	Allows model to read ORM objects	True
json_encoders	Custom JSON serialization for types	{datetime: lambda v: v.isoformat()}
allow_population_by_field_name	Allow using field names in input	True
anystr_strip_whitespace	Strip whitespace from strings	True

Key Takeaways

Use a nested Config class inside your Pydantic model to customize behavior.

Set orm_mode = True to enable compatibility with ORM objects in FastAPI.

Use json_encoders in Config to customize JSON output for special types.

Forgetting orm_mode causes errors when returning ORM instances as responses.

Config options help tailor validation and serialization to your app needs.