How to flatten objects with attribute-style access?

[tokarenko]

Please adivse how to flatten objects with attribute-style access?
Documentation says that name_mapping map supports dictionary-style access.
For example, I would like to map nested_attr of Nested in Source to the same of Flat object:

@dataclass
class Nested:
    nested_attr: str

@dataclass
class Source:
    container: dict[str, Nested]

@dataclass
class Flat:
    nested_attr: str

retort = ConversionRetort(
            recipe=[
                name_mapping(
                    Flat,
                    map = {
                        "nested_attr": ["container", "nested_key", "nested_attr"],
                    }
                ),
            ],
        )