Have you ever wondered why your SQLAlchemy objects suddenly have database IDs before you call commit()? Or why your queries sometimes see changes that you thought weren't saved yet? The answer lies in understanding flush() — SQLAlchemy's most powerful yet misunderstood operation. The flush () method is used to synchronize the in-memory state of objects with the database session. When you call flush (), SQLAlchemy generates SQL statements to insert, update, or delete objects that have been added, modified, or deleted within the current transaction. Specifically, the flush occurs before any individual SQL statement is issued as a result of a Query or a 2.0-style Session.execute() call, as well as within the Session.commit() call before the transaction is committed. Let SQLAlchemy auto-flush before queries or commits—manual flush() is rarely needed here. By mastering flush(), you’ll write more efficient, error-free SQLAlchemy code. The various methods mentioned in this article (commit / flush / expire / refresh / merge) are all slightly different ways to accomplish that last step in the lifecycle of persisting changes back to the database. An in-depth look at the fundamental differences between flush and commit in SQLAlchemy, focusing on usage scenarios, performance implications, and proper memory management. If you monitor when the various queries get executed, you'll see the UPDATEs don't hit the database until you call session.commit(). In some situations you might want to execute the UPDATE statements before issuing a COMMIT. In conclusion, flush () and commit () are two important methods in SQLAlchemy that are used to synchronize changes made to objects with the database. While flush () synchronizes the changes without committing them, commit () permanently saves the changes. session.flush() and session.commit() are both critical for managing transactions in SQLAlchemy, but they serve distinct roles: flush() synchronizes the session with the database, making changes visible within the transaction but not permanent. These operations do not have last over the database before commit, if the program is aborted in the transaction in the session, will lose any changes. Session uses Add to register your transaction, but you have not passed them to the database before Flush.