Returning fields computed/derived from values of other fields

First Check

• I added a very descriptive title to this issue.
• I used the GitHub search to find a similar issue and didn’t find it.
• I searched the FastAPI documentation, with the integrated search.
• I already searched in Google “How to X in FastAPI” and didn’t find any information.
• I already read and followed all the tutorial in the docs and didn’t find an answer.
• I already checked if it is not related to FastAPI but to Pydantic.
• I already checked if it is not related to FastAPI but to Swagger UI.
• I already checked if it is not related to FastAPI but to ReDoc.

Commit to Help

• I commit to help with one of those options 👆

Example Code

class MyModel(BaseModel):

    a: str
    b: str

    @property
    def c():
        return a + b

Description

I’m trying to make FastAPI return some extra fields as part of my JSON response which is based on the values of some other fields.

I can’t find any mention of this in the docs.

I don’t want to add the derived field and mutate the object whenever its dependencies update, as that seems a lot more error prone. Ideally I’d love to do something like in the example code snippet, where c is defined to always be a + b and this gets evaluated whenever a response is serialised.

Thanks!

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.63.0

Python Version

3.8.7

Additional Context

No response

[Norbo11]

@dylanwa indeed, I’m using root_validator and it’s working quite well:

from abc import abstractmethod

from pydantic import BaseModel, root_validator


class BaseModelWithStringRepresentation(BaseModel):

    string_representation: str = ""

    @classmethod
    @abstractmethod
    def get_string_representation(cls, values: dict):
        pass

    @root_validator
    def validate_string_representation(cls, values: dict):
        values["string_representation"] = cls.get_string_representation(values)
        return values

I have models subclassing BaseModelWithStringRepresentation and implementing get_string_representation where they will return a value based on some computation on their fields. Thanks @kaiix for the suggestion.

[ericman93]

what is the best solution for adding calculated fields from the DB? I have output model and it’s dict method I added fetches to return to the user but dict is sync method and my db queries can be async