Database session management in FastAPI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the correct function for creating a database session.

FastAPI

from sqlalchemy.orm import [1]

Drag options to blanks, or click blank then click option'

ASessionLocal

Bsessionmaker

Ccreate_engine

DBase

Attempts:

3 left

💡 Hint

Common Mistakes

Using create_engine instead of sessionmaker

Importing Base instead of sessionmaker

Confusing SessionLocal with sessionmaker

The sessionmaker function is used to create a new session factory in SQLAlchemy, which is essential for managing database sessions in FastAPI.

2fill in blank

medium

Complete the code to create a new database session instance.

FastAPI

db = [1]()

Drag options to blanks, or click blank then click option'

Aengine

BBase

Csessionmaker

DSessionLocal

Attempts:

3 left

💡 Hint

Common Mistakes

Calling engine() instead of SessionLocal()

Using sessionmaker() directly without configuring it

Trying to use Base as a session

SessionLocal is the session factory created by sessionmaker. Calling it returns a new session instance.

3fill in blank

hard

Fix the error in the dependency function to properly close the database session.

FastAPI

def get_db():
    db = SessionLocal()
    try:
        yield [1]
    finally:
        db.close()

Drag options to blanks, or click blank then click option'

ASessionLocal

Bengine

Cdb

DBase

Attempts:

3 left

💡 Hint

Common Mistakes

Yielding the session factory instead of the session instance

Yielding the engine or Base instead of the session

Not yielding anything

The function should yield the session instance db so that the caller can use it. Yielding SessionLocal or other objects is incorrect.

4fill in blank

hard

Fill both blanks to create a session factory with autocommit disabled and autoflush enabled.

FastAPI

SessionLocal = sessionmaker(bind=engine, autocommit=[1], autoflush=[2])

Drag options to blanks, or click blank then click option'

AFalse

BTrue

CNone

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Setting autocommit to True causing unexpected commits

Disabling autoflush leading to stale queries

Using None or 0 instead of boolean values

By default, autocommit should be False to control transactions manually, and autoflush is often set to True to flush changes automatically before queries.

5fill in blank

hard

Fill all three blanks to define a FastAPI dependency that provides a database session and ensures it is closed after use.

FastAPI

def get_db():
    db = [1]()
    try:
        yield [2]
    finally:
        [3].close()

Drag options to blanks, or click blank then click option'

ASessionLocal

Bdb

Dengine

Attempts:

3 left

💡 Hint

Common Mistakes

Yielding the session factory instead of the session instance

Closing the wrong variable like engine instead of the session

Not closing the session at all

The dependency function creates a session by calling SessionLocal(), yields the session db for use, and finally closes the session with db.close().

Practice

1. What is the main purpose of using a get_db function in FastAPI when working with databases?

easy

A. To create and close a database session for each request safely

B. To store user data permanently in memory

C. To handle HTTP requests directly without a database

D. To generate HTML templates for responses

Solution

1. Step 1: Understand the role of get_db

   The get_db function is designed to open a database session when a request starts and close it when the request ends.

2. Step 2: Recognize safe database session management

   This ensures that each request has its own session, preventing conflicts and resource leaks.

3. Final Answer:

   To create and close a database session for each request safely -> Option A

4. Quick Check:

   Database session management = create and close session [OK]

Hint: Remember: get_db opens and closes sessions per request [OK]

Common Mistakes:

• Thinking get_db stores data permanently
• Confusing get_db with HTTP request handling
• Assuming get_db generates HTML

2. Which of the following is the correct way to declare a dependency for a database session in a FastAPI route using Depends?

easy

A. def read_items(db = Depends(Session)):

B. def read_items(db: get_db = Session()):

C. def read_items(db: Session = get_db()):

D. def read_items(db: Session = Depends(get_db)):

Solution

1. Step 1: Understand FastAPI dependency injection syntax

   FastAPI uses Depends to inject dependencies like database sessions into route functions.

2. Step 2: Correct syntax for session injection

   The correct syntax is to type hint the parameter as Session and assign it Depends(get_db) to call the dependency function.

3. Final Answer:

   def read_items(db: Session = Depends(get_db)): -> Option D

4. Quick Check:

   Dependency injection = parameter: Type = Depends(function) [OK]

Hint: Use parameter: Type = Depends(function) for dependencies [OK]

Common Mistakes:

• Calling get_db() directly in parameter default
• Using Depends with a class instead of a function
• Swapping parameter and default values

3. Given this FastAPI route code snippet, what will be the output if the database session is correctly managed?

from fastapi import FastAPI, Depends
from sqlalchemy.orm import Session

app = FastAPI()

def get_db():
    db = Session()
    try:
        yield db
    finally:
        db.close()

@app.get('/items')
def read_items(db: Session = Depends(get_db)):
    items = db.query(Item).all()
    return items

medium

A. An error because the session is not closed

B. An empty list because the query is missing

C. A list of all items from the database

D. A syntax error due to wrong yield usage

Solution

1. Step 1: Analyze the get_db function behavior

   The get_db function creates a session, yields it for use, then closes it safely after the request.

2. Step 2: Understand the route's database query

   The route uses the session to query all Item records and returns them as a list.

3. Final Answer:

   A list of all items from the database -> Option C

4. Quick Check:

   Yielded session + query = list of items [OK]

Hint: Yielded session allows safe query and close after use [OK]

Common Mistakes:

• Assuming session is not closed causing error
• Thinking yield causes syntax error
• Believing query returns empty without data

4. Identify the error in this FastAPI database session management code:

def get_db():
    db = Session()
    yield db
    db.close()

@app.post('/add')
def add_item(item: Item, db: Session = Depends(get_db)):
    db.add(item)
    db.commit()

medium

A. The item parameter should be inside get_db

B. The session is closed after yield, so it may not close if an exception occurs

C. The Depends is used incorrectly in the route

D. The db.commit() is missing

Solution

1. Step 1: Review session closing in get_db

   The db.close() is called after yield without a try-finally block, so if an exception happens, the session may never close.

2. Step 2: Understand proper session cleanup

   Using try-finally ensures the session closes even if errors occur during request handling.

3. Final Answer:

   The session is closed after yield, so it may not close if an exception occurs -> Option B

4. Quick Check:

   Session close needs try-finally for safety [OK]

Hint: Always use try-finally to close sessions safely [OK]

Common Mistakes:

• Ignoring try-finally for session cleanup
• Forgetting to commit changes
• Misplacing Depends usage

5. You want to ensure that your FastAPI app's database sessions are properly managed and that any changes are committed only if no exceptions occur. Which of the following get_db implementations best achieves this?

hard

A. def get_db():
db = Session()
try:
yield db
db.commit()
except:
db.rollback()
raise
finally:
db.close()

B. def get_db():
db = Session()
yield db
db.commit()
db.close()

C. def get_db():
db = Session()
try:
yield db
finally:
db.close()

D. def get_db():
db = Session()
yield db
db.rollback()
db.close()

Solution

1. Step 1: Understand transaction management needs

   We want to commit changes only if no errors occur, otherwise rollback to avoid partial changes.

2. Step 2: Analyze each get_db implementation

   def get_db(): db = Session() try: yield db db.commit() except: db.rollback() raise finally: db.close() uses try-except-finally to commit on success, rollback on error, and always close the session, which is the safest approach.

3. Final Answer:

   def get_db(): db = Session() try: yield db db.commit() except: db.rollback() raise finally: db.close() -> Option A

4. Quick Check:

   Commit on success, rollback on error, always close [OK]

Hint: Use try-except-finally to commit, rollback, and close sessions [OK]

Common Mistakes:

• Committing after yield without error handling
• Not rolling back on exceptions
• Closing session without try-finally