[Python] How to build Pydantic models that do not produce nesting on serialization?

I am looking for a way to build a model Foo below, where serializing does not produce nesting (and of course Foo still does validation)

Desired behaviour: bar is validated according to Foo.

y = Bar(bar=Foo('xxx'))


Desired output:

print(y.json())
# {"bar": "xxx"}

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