Add support for lazy connections

[RobertCraigie]

Problem

Currently, defining multiple event handlers using a framework that doesn't support a callback like on_startup() is annoying as you have to check if the client is connected or not before making a query.

Suggested solution

Refactor the internal engine connection to be lazy, i.e. whenever a query is made, connect to the database before running the query.

[RobertCraigie]

We need to take special care that concurrent queries do not create multiple connections.

[AdeelK93]

could you do this with a global?

[RobertCraigie]

@AdeelK93 sorry what do you mean?

[AdeelK93]

something like this - creates a connection if one does not exist, tracking prior connection globally

from prisma import Prisma
_db = None

async def get_db() -> Prisma:
    global _db
    if not _db:
        _db = Prisma(auto_register=True)
        await _db.connect()
    return _db

[RobertCraigie]

yeah that'd work but you'd also want to make sure you don't create multiple clients if a request comes in while you're connecting

e.g. something like this (untested)

import asyncio
from prisma import Prisma
from typing import Optional

_db: Optional[Prisma] = None
_lock = asyncio.Lock()

async def get_db() -> Prisma:
    global _db
    if _db is None:
        async with _lock:
            if _db is None:
                db = Prisma(auto_register=True)
                await db.connect()
                _db = db
    return _db