"""数据库引擎与会话工厂（SQLite 本地过渡 / PostgreSQL 生产）。"""
from __future__ import annotations

from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
from sqlalchemy.orm import DeclarativeBase

from config import get_settings


class Base(DeclarativeBase):
    pass


_engine = None
_session_factory = None


def get_engine():
    global _engine
    if _engine is None:
        settings = get_settings()
        connect_args: dict = {}
        # SQLite 需允许多线程/多协程访问同一文件
        if settings.database_url.startswith("sqlite"):
            connect_args["check_same_thread"] = False
        _engine = create_async_engine(
            settings.database_url,
            echo=False,
            future=True,
            connect_args=connect_args,
        )
    return _engine


def get_session_factory() -> async_sessionmaker[AsyncSession]:
    global _session_factory
    if _session_factory is None:
        _session_factory = async_sessionmaker(
            get_engine(),
            class_=AsyncSession,
            expire_on_commit=False,
        )
    return _session_factory


async def get_db():
    """FastAPI 依赖：请求级 AsyncSession。"""
    factory = get_session_factory()
    async with factory() as session:
        yield session