第一步

backend/app/core/db.py

@@ -0,0 +1,76 @@
+"""SQLAlchemy 2.x 同步引擎 + 会话管理（MVP: SQLite）。
+
+切换 PostgreSQL 时（扩展接口 1）：
+1. DATABASE_URL 改为 postgresql+asyncpg://...
+2. 引擎换 create_async_engine + async_sessionmaker
+3. get_session 改 async generator + yield
+4. 业务层 Repository 调用加 await
+"""
+
+from collections.abc import Generator
+
+from sqlalchemy import create_engine
+from sqlalchemy.orm import Session, sessionmaker
+
+from app.core.config import get_settings
+
+_engine = None
+_session_factory: sessionmaker[Session] | None = None
+
+
+def get_engine():
+    global _engine
+    if _engine is None:
+        settings = get_settings()
+        _engine = create_engine(
+            settings.database_url,
+            pool_pre_ping=True,
+            echo=False,
+            # SQLite 专属：启用 WAL 模式（并发读 + 写串行化）
+            connect_args={"check_same_thread": False} if "sqlite" in settings.database_url else {},
+        )
+        # SQLite: 启用 WAL 模式与外键约束
+        if "sqlite" in settings.database_url:
+            from sqlalchemy import event, text
+
+            @event.listens_for(_engine, "connect")
+            def _set_sqlite_pragma(dbapi_conn, _):  # type: ignore[no-untyped-def]
+                cursor = dbapi_conn.cursor()
+                cursor.execute("PRAGMA journal_mode=WAL")
+                cursor.execute("PRAGMA foreign_keys=ON")
+                cursor.close()
+
+    return _engine
+
+
+def get_session_factory() -> sessionmaker[Session]:
+    global _session_factory
+    if _session_factory is None:
+        _session_factory = sessionmaker(
+            get_engine(),
+            class_=Session,
+            expire_on_commit=False,
+        )
+    return _session_factory
+
+
+def get_session() -> Generator[Session, None, None]:
+    """FastAPI 依赖：请求级会话。"""
+    factory = get_session_factory()
+    session = factory()
+    try:
+        yield session
+        session.commit()
+    except Exception:
+        session.rollback()
+        raise
+    finally:
+        session.close()
+
+
+def dispose_engine() -> None:
+    global _engine, _session_factory
+    if _engine is not None:
+        _engine.dispose()
+    _engine = None
+    _session_factory = None