Creating tables (db.create_all) in Flask - Practice Exercises

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when you call db.create_all() in Flask?

Consider a Flask app using SQLAlchemy. What does db.create_all() do when executed?

Flask

from flask_sqlalchemy import SQLAlchemy
from flask import Flask

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

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

with app.app_context():
    db.create_all()

A It drops all tables and then creates them again.

B It deletes all existing tables and recreates them empty.

C It only creates tables if the database is empty, otherwise does nothing.

D It creates all tables defined by the models in the connected database if they don't exist.

💡 Hint

Think about what create_all() means literally and how it behaves with existing tables.

db.create_all() creates tables for all models that do not yet exist in the database. It does not drop or delete existing tables.

📝 Syntax

intermediate

Identify the correct way to call db.create_all() in Flask

Which of the following code snippets correctly calls db.create_all() in a Flask app context?

Flask

from flask_sqlalchemy import SQLAlchemy
from flask import Flask

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///test.db'
db = SQLAlchemy(app)

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

A app.app_context().create_all(db)

B db.create_all(app.app_context())

C

with app.app_context():
    db.create_all()

D db.create_all()

💡 Hint

Remember that db.create_all() needs to run inside the Flask app context.

Calling db.create_all() requires the Flask app context to be active. Using with app.app_context(): ensures this.

🔧 Debug

advanced

Why does db.create_all() not create tables for new models?

You added a new model to your Flask app but db.create_all() does not create its table. What is the most likely cause?

Flask

class Order(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    item = db.Column(db.String(100))

# After adding this, you run:
with app.app_context():
    db.create_all()

A The primary key is missing in the new model.

B The new model class was not imported before calling <code>db.create_all()</code>.

C You need to call <code>db.create_all(force=True)</code> to create new tables.

D The database URI is incorrect, so tables cannot be created.

💡 Hint

Think about how SQLAlchemy knows about models when creating tables.

SQLAlchemy only creates tables for models it knows about. If the new model is not imported or registered before db.create_all(), its table won't be created.

❓ state_output

advanced

What is the state of the database after calling db.create_all() twice?

Given a Flask app with a model User, what happens if you call db.create_all() two times in a row inside the app context?

Flask

with app.app_context():
    db.create_all()
    db.create_all()

A The tables are created once; the second call does nothing since tables exist.

B The tables are dropped and recreated on the second call.

C An error is raised on the second call because tables already exist.

D The tables are duplicated, creating multiple copies.

💡 Hint

Consider how create_all() behaves with existing tables.

db.create_all() creates tables only if they do not exist. Calling it multiple times does not recreate or drop tables.

🧠 Conceptual

expert

Why should db.create_all() be called inside app.app_context()?

Explain why db.create_all() must be called inside with app.app_context(): in a Flask application.

A Because Flask needs the app context to access configuration and database bindings during table creation.

B Because calling it outside app context causes the database to reset.

C Because <code>db.create_all()</code> modifies the app's routing rules which require context.

D Because it only works inside a request context, not app context.

💡 Hint

Think about what the app context provides to Flask extensions like SQLAlchemy.

The app context provides access to the app's configuration and database connection. Without it, db.create_all() cannot access the database properly.