Addresses#421

Currently, if you return aBaseModel instance in a route with aBaseModel response_type, theskip_defaults value does not influence the result. This PR fixes this.

Details

The source of the problem is that callingfield.validate on a model results in a new model with the fields set. I dug in on the pydantic side and it looks like this is due to the recent change to re-parse the model with a non-subclass type when response_model is set. If you look inBaseModel.validate (which is ultimately called if you follow it deep enough), you can see the precise spot where defaults aren't being skipped:

@classmethoddefvalidate(cls:Type['Model'],value:Any)->'Model':ifisinstance(value,dict):returncls.parse_obj(value)elifisinstance(value,cls):returnvalue.copy()elifcls.__config__.orm_mode:returncls.from_orm(value)else:withchange_exception(DictError,TypeError,ValueError):returncls.parse_obj(value)

It calls intoparse_obj on the last line, which subsequently callsdict on the object and then parses the dict. Unfortunately, this means defaults don't get skipped.

I don't think it makes sense to change this on the pydantic side because otherwise you'd run into problems parsing models with default values into non-subclass models with required values. So, if we want to stick with this approach of creating a new class to force a reparse, we probably need to handle the

That said, I would also be in favor of a larger refactor if we could come up with a way to get the desired field safely without another round of parsing/dumping 😬. But that does seem hard.