SQLAlchemy async sessions forbid implicit IO, so a lazy load raises MissingGreenlet

With AsyncSession, any attribute access that would emit SQL outside an await raises sqlalchemy.exc.MissingGreenlet, reporting that greenlet_spawn has not been called and await_only cannot be called here. That covers lazy-loaded relationships, deferred columns, and objects expired by a commit, which is why AsyncSession is almost always constructed with expire_on_commit set to False.

Relationships must be loaded up front with selectinload as a query option or lazy set to selectin on the relationship itself. To touch an unloaded attribute deliberately, await session.refresh naming the attribute, or wrap the access in session.run_sync. The same error appears when an ORM object is serialised after the session has closed, so the boundary has to be explicit: build response objects while the session is still open.