DB sessions

A session is a unit of work — track changes, commit, rollback. One per request.

Code example

db = SessionLocal()
try:
    user = User(username='ada')
    db.add(user)
    db.commit()         # writes to DB
    db.refresh(user)    # loads the new id
except:
    db.rollback()       # undoes partial changes
finally:
    db.close()

How it works

A DB session is a holding area for objects you're working with. When you `db.add(obj)`, the row isn't written yet — it's tracked in memory. `db.commit()` flushes all pending changes to the DB in a single transaction. If anything goes wrong, `db.rollback()` undoes everything since the last commit. The `get_db()` dependency wraps this in try/finally so every request gets a fresh session and the session is always closed, even on exception.