Flask-SQLAlchemy setup - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this Flask-SQLAlchemy model query?

Given this Flask app and model setup, what will User.query.count() return after adding two users and committing?

Flask

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///:memory:'
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False

db = SQLAlchemy(app)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50), nullable=False)

with app.app_context():
    db.create_all()
    user1 = User(name='Alice')
    user2 = User(name='Bob')
    db.session.add(user1)
    db.session.add(user2)
    db.session.commit()
    count = User.query.count()

A 2

B 0

C 1

D Raises an error because of missing app context

💡 Hint

Remember that db.create_all() and queries require the app context.

After adding two users and committing, User.query.count() returns 2 because both users are saved in the database.

📝 Syntax

intermediate

Which option correctly initializes Flask-SQLAlchemy with an app factory?

Choose the correct way to set up Flask-SQLAlchemy when using an app factory pattern.

Flask

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

db = SQLAlchemy()

def create_app():
    app = Flask(__name__)
    app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///test.db'
    # Initialize db here
    return app

A db.init_app(app) inside create_app before return

B db = SQLAlchemy(app) inside create_app

C db.create_all() inside create_app before return

D app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///test.db' after return

💡 Hint

When using app factory, you create the db object outside and initialize it inside.

In app factory pattern, db = SQLAlchemy() is outside, and db.init_app(app) is called inside create_app() before returning the app.

🔧 Debug

advanced

Why does this Flask-SQLAlchemy code raise an error?

What error does this code raise and why?

Flask

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///:memory:'
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False

db = SQLAlchemy(app)

class Product(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(80), nullable=False)

product = Product(name='Book')
db.session.add(product)
db.session.commit()

A Raises AttributeError: 'Product' object has no attribute 'name'

B Raises IntegrityError due to missing primary key

C Raises RuntimeError: Working outside of application context

D No error, commits successfully

💡 Hint

Check if the code runs inside the Flask app context.

The code tries to add and commit outside the Flask app context, causing a RuntimeError.

❓ state_output

advanced

What is the value of user.email after this code runs?

Consider this Flask-SQLAlchemy model and code snippet. What will user.email be?

Flask

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///:memory:'
app.config['SQLALCHEMY_TRACK_MODIFICATIONS'] = False

db = SQLAlchemy(app)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(50), nullable=False)
    email = db.Column(db.String(120), unique=True, nullable=True)

with app.app_context():
    db.create_all()
    user = User(username='john')
    db.session.add(user)
    db.session.commit()
    user = User.query.first()

A Raises AttributeError

B None

C Empty string ''

D '[e-mail]'

💡 Hint

Check the default value of email when not set.

The email column is nullable and not set, so its value is None.

🧠 Conceptual

expert

Which statement best describes Flask-SQLAlchemy's session behavior?

Choose the most accurate description of how Flask-SQLAlchemy manages database sessions.

A It shares a single global session object across all threads without isolation.

B It creates a new session for each query and closes it immediately after.

C It requires manual session creation and closing for every database operation.

D It uses a scoped session tied to the Flask app context to handle concurrent requests safely.

💡 Hint

Think about how Flask handles multiple requests and how SQLAlchemy sessions should behave.

Flask-SQLAlchemy uses a scoped session that is tied to the Flask app context, ensuring thread safety and proper session lifecycle per request.