Best way to exclude unset fields from nested FastAPI model response

Assuming I have Pydantic-based FastAPI models similar to:

class Filter(BaseModel, extra=Extra.forbid):
    expression: str | None
    kind: str | None
    ...

class Configuration(BaseModel):
    filter: Filter | None = {}
    ....

By default a model that is sent nothing for the filter will end up exporting like this:

{
   "filter": {
       "expression": None,
       "kind": None,
       ...
   },
   ...
}

What I want instead is this:

{
   "filter": {},
   ...
}

What is the best way to have the nested model always have the exclude_unset behavior when exporting?

Sometimes in a complicated model I want some nested models to exclude unset fields but other ones to include them. I'm aware of exclude_unset and response_model_exclude_unset, but both affect the entire model.

It feels like there should be a way to specify behavior for a nested model and have the be consistently respected, but I can't find how to do this. Is there a better way to approach this?

source https://stackoverflow.com/questions/76105056/best-way-to-exclude-unset-fields-from-nested-fastapi-model-response