FastAPI SQLAlchemy Tutorial: Sessions, Flushes and ORM Relationships
SQLAlchemy's ORM maps Python objects to relational data and coordinates changes through a session. The session tracks objects and participates in transactions; it is not simply another name for the database engine. A flush sends pending changes to the database within the transaction, while commit completes that transaction. Relationships introduce further questions about when related data is loaded and whether it can be accessed after the session is closed. Integrating this behavior with FastAPI requires a clear session boundary and suitable cleanup. The key learning task is understanding ORM state and transaction behavior rather than just placing queries inside routes.
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Why Use FastAPI SQLAlchemy Tutorial: Sessions, Flushes and ORM Relationships
- Distinguish an engine from a session and its transaction
- Explain why flushing does not equal committing
- Scope ORM work to an explicit unit of work
- Load relationships before they become unavailable
- Recover correctly from failed writes and release session resources
Frequently Asked Questions
How does a SQLAlchemy session differ from an engine?
An engine manages database connectivity and pooling, while a session coordinates ORM objects and their transactional work. Several sessions can use an engine, but their object state and transaction lifecycles should not be treated as one shared unit.
What is the difference between flush and commit?
Flush sends pending ORM changes as database statements within the current transaction. Commit completes the transaction and generally flushes pending work first. Flushed changes can still be rolled back before the transaction is committed.
Why avoid sharing one session across unrelated requests?
A session contains mutable state and transactional context. Sharing it can mix independent operations and concurrent activity, so use an appropriate separate session scope rather than treating a global session as a universal connection.
What can go wrong with lazy-loaded relationships in a response?
Accessing an unloaded relationship can require additional database work. If the session is closed or the execution context does not permit that access, serialization can fail, so decide how required data will be loaded before constructing output.
What should happen after a flush raises an error?
A failed flush leaves the session needing rollback before further use in its normal transaction flow. Handle the exception and restore or end the unit of work, rather than continuing queries as if the failure had no effect.