langchain/chains/loading.py/_load_vector_db_qa
def _load_vector_db_qa(config: dict, **kwargs: Any) -> VectorDBQA:
    if "vectorstore" in kwargs:
        vectorstore = kwargs.pop("vectorstore")
    else:
        raise ValueError("`vectorstore` must be present.")
    if "combine_documents_chain" in config:
        combine_documents_chain_config = config.pop("combine_documents_chain")
        combine_documents_chain = load_chain_from_config(combine_documents_chain_config)
    elif "combine_documents_chain_path" in config:
        combine_documents_chain = load_chain(config.pop("combine_documents_chain_path"))
    else:
        raise ValueError(
            "One of `combine_documents_chain` or "
            "`combine_documents_chain_path` must be present."
        )
    return VectorDBQA(
        combine_documents_chain=combine_documents_chain,
        vectorstore=vectorstore,
        **config,
    )