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

Question

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"}

Answer 1

You can use RootModel[str] for this.

See this example:

from pydantic import *

class Foo(RootModel[str]):
    pass

class Bar(BaseModel):
    bar: Foo

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

print(y.model_dump_json())

That way, you can have validation of Foo without having the nesting as you describe. That said, using Annotated here would probably make more sense.