# File: backend/app/db/database.py (New - Database setup)
# Description: SQLAlchemy 数据库引擎和会话设置

from sqlalchemy.ext.asyncio import create_async_engine, AsyncSession
from sqlalchemy.orm import sessionmaker, declarative_base
from app.core.config import DATABASE_URL

# 创建异步数据库引擎
# connect_args={"check_same_thread": False} is needed only for SQLite.
# It's not needed for other databases.
engine = create_async_engine(DATABASE_URL, echo=True, connect_args={"check_same_thread": False})

# 创建异步会话工厂
# expire_on_commit=False prevents attributes from expiring after commit.
AsyncSessionFactory = sessionmaker(
    bind=engine,
    class_=AsyncSession,
    expire_on_commit=False,
    autocommit=False,
    autoflush=False,
)

# 创建数据模型的基础类
Base = declarative_base()

# --- Dependency to get DB session ---
async def get_db_session() -> AsyncSession:
    """FastAPI dependency to get an async database session."""
    async with AsyncSessionFactory() as session:
        try:
            yield session
            await session.commit() # Commit transaction if successful
        except Exception:
            await session.rollback() # Rollback on error
            raise
        finally:
            await session.close()

# --- Function to create tables (call this on startup) ---
async def create_db_and_tables():
    async with engine.begin() as conn:
        # await conn.run_sync(Base.metadata.drop_all) # Use drop_all carefully in dev
        await conn.run_sync(Base.metadata.create_all)