FastAPI - GET request results in typeerror (value is not a valid dict)

Question

this is my database schema.

I defined my Schema like this:

from pydantic import BaseModel

class Userattribute(BaseModel):
    name: str
    value: str
    user_id: str
    id: str

This is my model:

class Userattribute(Base):
    __tablename__ = "user_attribute"

    name = Column(String)
    value = Column(String)
    user_id = Column(String)
    id = Column(String, primary_key=True, index=True)

In a crud.py I define a get_attributes method.

def get_attributes(db: Session, skip: int = 0, limit: int = 100):
    return db.query(models.Userattribute).offset(skip).limit(limit).all()

This is my GET endpoint:

@app.get("/attributes/", response_model=List[schemas.Userattribute])
def read_attributes(skip: int = 0, limit: int = 100, db: Session = Depends(get_db)):
    users = crud.get_attributes(db, skip=skip, limit=limit)
    print(users)
    return users

The connection to the database seems to work, but a problem is the datatype:

pydantic.error_wrappers.ValidationError: 7 validation errors for Userattribute
response -> 0
  value is not a valid dict (type=type_error.dict)
response -> 1
  value is not a valid dict (type=type_error.dict)
response -> 2
  value is not a valid dict (type=type_error.dict)
response -> 3
  value is not a valid dict (type=type_error.dict)
response -> 4
  value is not a valid dict (type=type_error.dict)
response -> 5
  value is not a valid dict (type=type_error.dict)
response -> 6
  value is not a valid dict (type=type_error.dict)

Why does FASTApi expect a dictionary here? I don´t really understand it, since I am not able to even print the response. How can I fix this?

Answer 1

Pydantic 2 changed how models gets configured, so if you're using the most recent version of Pydantic, see the section named Pydantic 2 below.

SQLAlchemy does not return a dictionary, which is what pydantic expects by default. You can configure your model to also support loading from standard orm parameters (i.e. attributes on the object instead of dictionary lookups):

class Userattribute(BaseModel):
    name: str
    value: str
    user_id: str
    id: str

    class Config:
        orm_mode = True

You can also attach a debugger right before the call to return to see what's being returned.

Since this answer has become slightly popular, I'd like to also mention that you can make orm_mode = True the default for your schema classes by having a common parent class that inherits from BaseModel:

class OurBaseModel(BaseModel):
    class Config:
        orm_mode = True


class Userattribute(OurBaseModel):
    name: str
    value: str
    user_id: str
    id: str

This is useful if you want to support orm_mode for most of your classes (and for those where you don't, inherit from the regular BaseModel).

Pydantic 2

Pydantic 2 has replaced the internal Config class with a model_config field:

from pydantic import ConfigDict

class OurBaseModel(BaseModel):
    model_config = ConfigDict(from_attributes=True)

This works in the same way as the old orm_mode.