def ensure_model_obj_has_mapping_constructor(model_obj):
    """
    Ensure that `model_obj` has a 'get_mappings_for_fit' method. Raises a
    helpful ValueError if otherwise.
    """
    if not hasattr(model_obj, 'get_mappings_for_fit'):
        msg = "model_obj MUST have a 'get_mappings_for_fit' method."
        raise ValueError(msg)
    return None
