"""async SQLAlchemy 引擎与会话。"""
from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
from sqlalchemy.orm import DeclarativeBase

from .config import settings


class Base(DeclarativeBase):
    """所有 ORM 模型的基类。"""


# echo=False；远程 PG 的空闲连接可能被中间层断开，pre_ping + recycle 自动剔除死连接
# pool_size/max_overflow：preview 冷路径每请求开 2 个会话（2 波并发），Redis 故障时全请求变冷，
# 默认 5+10 不够并发冷请求用（连接池满了只会排队，不会崩）
engine = create_async_engine(
    settings.database_url, echo=False, future=True,
    pool_pre_ping=True, pool_recycle=1800,
    pool_size=10, max_overflow=20,
)
async_session = async_sessionmaker(engine, expire_on_commit=False, class_=AsyncSession)


async def get_session() -> AsyncSession:
    """FastAPI 依赖：提供一个事务会话。"""
    async with async_session() as session:
        yield session