Create a reference builder with specified relative location.
    using getter.relative_ref("some", "submodel") to get a value with key
    "toto" will gives reference.Relative("some", "submodel", "toto")

def relative_ref(*parts):
    """
    Create a reference builder with specified relative location.
    using getter.relative_ref("some", "submodel") to get a value with key
    "toto" will gives reference.Relative("some", "submodel", "toto")
    """

    def relative_ref(_value, context, **_params):
        return reference.Relative(*(parts + (context["key"], )))

    return relative_ref