DetachedInstanceError when accessing lazy-loaded relationship after session close

Problem Context

When a SQLAlchemy Session is closed, accessing a lazy-loaded relationship on an object raises sqlalchemy.orm.exc.DetachedInstanceError.

Goal

Determine whether the occurrence of DetachedInstanceError signals a performance bottleneck in session handling before applying optimizations such as eager loading or session configuration changes.

Constraints & Uncertainty

The error appears only after the session is terminated, but it is unclear if the underlying issue is merely the session lifecycle or if it reflects excessive lazy-load queries that could be optimized. Additionally, the impact of expire_on_commit=False on the error frequency is uncertain.

Specific Questions

• Does the frequency of DetachedInstanceError correlate with increased latency for lazy-loaded relationships?
• Can instrumentation of query execution before session close reliably identify which lazy loads contribute most to the error?
• What combination of session options (e.g., expire_on_commit, joinedload) balances error avoidance with query performance?