PostgresChatMessageHistory API break the usage of LCEL and Langserv

[pprados]

The constructor of PostgresChatMessageHistory accepts only sync or async connection, and not an Engine.

First, the connection was open in get_session_history, and pending during the life cycle of RunnableWithMessageHistory.

So, it's impossible to use it in a "singleton" approach.

With langserv, you must declare the API with

add_routes(
    app,
    chain,
    path="/",
)

You must have a global variable chain.

chat_chain = RunnableWithMessageHistory(
    _context_and_question | retriever_chain,
    get_session_history=get_session_history, # <-- here
    input_messages_key="question",
    history_messages_key="chat_history",
)

The get_session_history cannot be async. The PostgresChatMessageHistory.async_connection() can not be used.

PostgresChatMessageHistory must be used if the singleton form, to be use with langserv. The engine must be set, without connection.

[eyurtsev]

I'm hoping to avoid using too much SQLAlchemy. Would a psycopg pool object work for your use case?

[pprados]

Yes. The idea is to use the same approach as for the other integrations with sqlalchemy, exploiting the connection pool and sharing sessions if possible.

the vectorstore proposes

        connection: Union[None, DBConnection, Engine, AsyncEngine, str] = None,

I think this is preferable and usable.

@eyurtsev , I can propose a PR for that, I can propose a PR for this, but I don't want it to remain on hold for several weeks, as with my other PRs.

[pprados]

@eyurtsev
I propose a PR to resolve this problem.

[pprados]

A patch is being applied here