def extern_project_multi(self, context_handle, val, field_str_ptr, field_str_len):
    """Given a Key for `obj`, and a field name, project the field as a list of Keys."""
    c = self._ffi.from_handle(context_handle)
    obj = c.from_value(val[0])
    field_name = self.to_py_str(field_str_ptr, field_str_len)

    return c.vals_buf(tuple(c.to_value(p) for p in getattr(obj, field_name)))
Given a Key for `obj`, and a field name, project the field as a list of Keys.