5. Interview Questions- Using the Session
Source: https://docs.sqlalchemy.org/en/20/orm/session.html
1. What is the purpose of the Session in SQLAlchemy ORM?
Session in SQLAlchemy ORM?Answer:
The Session is the ORM’s interface to the database. It manages object persistence, transactions, and connection resources.
2. How do you create a new session in SQLAlchemy 2.0?
Answer:
from sqlalchemy.orm import Session
with Session(engine) as session:
...3. What is the benefit of using a with block for Session?
with block for Session?Answer: It ensures that the session is closed automatically, even if an error occurs—helping manage resources safely.
4. How do you persist a new object using the session?
Answer:
new_user = User(name="Alice")
session.add(new_user)
session.commit()5. What’s the difference between add() and add_all() in a session?
add() and add_all() in a session?Answer:
add(obj): Adds a single object to the session.add_all([obj1, obj2]): Adds multiple objects in one call.
6. What does session.commit() do?
session.commit() do?Answer: It flushes all pending changes to the database and commits the current transaction.
7. What’s the difference between session.commit() and session.flush()?
session.commit() and session.flush()?Answer:
flush(): Sends SQL to the DB but doesn’t commit the transaction.commit(): Flushes changes and commits the transaction.
8. What does session.rollback() do?
session.rollback() do?Answer: It rolls back the current transaction, reverting uncommitted changes in the database and expiring ORM objects.
9. What happens if you close a session with session.close()?
session.close()?Answer: It ends the session’s connection to the DB. ORM objects remain usable but detached (can’t issue new queries).
10. What is the difference between a “dirty” and a “new” object in the session?
Answer:
New: Not yet flushed to the DB (
session.new).Dirty: Modified but not committed (
session.dirty).
11. How do you delete an object from the database?
Answer:
12. What is expire_on_commit=True and when should you disable it?
expire_on_commit=True and when should you disable it?Answer:
It invalidates all ORM objects on commit(). You may disable it when you want objects to retain values after commit.
13. How can you inspect the state of the session?
Answer:
14. What happens if you access a flushed but uncommitted object’s attribute?
Answer: You get the latest value; SQLAlchemy automatically populates values returned by the flush (e.g., autogenerated IDs).
15. Can you reuse a session across multiple requests in a web app?
Answer: No. Sessions should be short-lived and scoped to a single unit of work or request.
16. How do you refresh an object with values from the DB?
Answer:
17. What is the autoflush parameter in a session?
autoflush parameter in a session?Answer:
It controls whether SQLAlchemy flushes changes automatically before a query. Defaults to True.
18. How do you merge a detached instance back into a session?
Answer:
19. What does session.get(Model, primary_key) do?
session.get(Model, primary_key) do?Answer:
It retrieves an object by primary key or returns None if not found.
20. What is the binds parameter in session configuration?
binds parameter in session configuration?Answer: It allows you to map different tables to different engines within the same session.
Here are 20 more SQLAlchemy ORM interview questions and answers based on the Session documentation (v2.0), focusing on identity management, advanced session operations, scoping, and transactional behavior.
21. What is the identity map in a SQLAlchemy session?
Answer: It's an in-memory cache where each object is keyed by its primary key. Within a session, the same object is never loaded twice.
22. What happens when you query the same object twice within one session?
Answer: The session returns the same Python object instance from its identity map.
23. How do you remove an object from the session without deleting it from the database?
Answer:
24. What does session.clear() do?
session.clear() do?Answer: It removes all objects from the session, including pending changes and identity tracking.
25. What is a detached object in SQLAlchemy?
Answer: An object that was once attached to a session but is no longer — either because the session was closed or it was expunged.
26. What happens if you modify a detached object and call session.commit()?
session.commit()?Answer:
Nothing unless you add() or merge() it back into the session first.
27. How can you tell if an object is persistent, pending, or detached?
Answer:
28. What’s the difference between Session.commit() and Session.flush() in terms of transaction boundaries?
Session.commit() and Session.flush() in terms of transaction boundaries?Answer:
flush()sends SQL but does not commit the transaction.commit()ends the current transaction and starts a new one.
29. Can you roll back only part of a session's changes?
Answer: No. SQLAlchemy rolls back the entire transaction. You’d need separate transactions or savepoints to isolate changes.
30. What is a sessionmaker and why is it useful?
Answer:
It’s a factory that produces new Session instances, useful for consistent session configuration.
31. What is the difference between Session() and sessionmaker()?
Session() and sessionmaker()?Answer:
Session()creates a session directly.sessionmaker()is a configurable session factory, ideal for reuse.
32. How do you manually bind a Session to a specific engine?
Session to a specific engine?Answer:
33. What’s the recommended practice for session management in web apps?
Answer: Use short-lived sessions tied to request/response cycles, often managed via dependency injection or middleware.
34. Can a session span multiple transactions?
Answer: Yes, a session can commit/rollback multiple times. Each commit or rollback defines a new transaction.
35. What happens if you try to use a session after it's been closed?
Answer: Most operations will fail with an error, especially queries and flushes.
36. What is autocommit and should it be used?
autocommit and should it be used?Answer:
autocommit=True is deprecated in SQLAlchemy 2.0. Explicit transaction control is the preferred approach.
37. Can multiple threads share the same session instance?
Answer: No. Sessions are not thread-safe. Use a separate session per thread or request.
38. How does Session.begin() differ from using with Session()?
Session.begin() differ from using with Session()?Answer:
session.begin() defines an explicit transaction block within an existing session scope.
39. How do you execute raw SQL inside a session?
Answer:
40. What’s the difference between commit() and flush() when using add()?
commit() and flush() when using add()?Answer:
add()stages the object.flush()sends it to the DB.commit()flushes + ends the transaction.
41. What does Session.begin() do?
Session.begin() do?Answer: It begins a new transaction explicitly. Used when you want to control the transaction scope inside a longer session lifecycle.
42. Can you nest transactions in SQLAlchemy ORM sessions?
Answer:
Yes, using savepoints via session.begin_nested().
43. What is the use of Session.in_transaction()?
Session.in_transaction()?Answer: It checks if the session is currently within a transaction block.
44. What is the Session.get_bind() method used for?
Session.get_bind() method used for?Answer:
Returns the Engine or Connection associated with the session or for a specific ORM-mapped class.
45. How does SQLAlchemy determine which engine to use for a query?
Answer:
Via the bind associated with the session or from binds configured during Session construction.
46. Can you change the bind for a session at runtime?
Answer:
Yes, using execution_options or by manually providing bind= to operations like Session.execute().
47. What does Session.is_active represent?
Session.is_active represent?Answer:
It returns True if the session has not been rolled back or invalidated due to errors.
48. What happens if an exception occurs during a session’s transaction block?
Answer: The session automatically rolls back the transaction and the session remains usable.
49. When should you use Session.rollback() explicitly?
Session.rollback() explicitly?Answer: When an exception occurs outside a context manager or when you want to discard uncommitted changes manually.
50. Why is using with Session(engine) as session: considered best practice?
with Session(engine) as session: considered best practice?Answer: It ensures:
Automatic cleanup with
.close()Safe transaction rollback
Clear scoping of database work
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