Lazy loading vs eager loading in Flask - Hands-On Comparison

Lazy Loading vs Eager Loading in Flask with SQLAlchemy

📖 Scenario: You are building a simple Flask web app to manage authors and their books. You want to understand how to load related data efficiently using SQLAlchemy's lazy loading and eager loading techniques.

🎯 Goal: Build a Flask app with SQLAlchemy models for Author and Book. Learn to set up relationships with lazy loading and eager loading, and see how they affect data fetching.

📋 What You'll Learn

Create SQLAlchemy models for Author and Book with a one-to-many relationship

Configure the relationship to use lazy loading

Change the relationship to use eager loading

Query authors and access their books to observe loading behavior

💡 Why This Matters

🌍 Real World

Web apps often need to load related data from databases efficiently. Choosing between lazy and eager loading helps optimize performance and user experience.

💼 Career

Understanding loading strategies is important for backend developers working with ORMs like SQLAlchemy to build scalable and fast web applications.

1

Set up SQLAlchemy models with lazy loading

Create SQLAlchemy models called Author and Book. Define a one-to-many relationship from Author to Book using books = relationship('Book', back_populates='author') with lazy loading (default). Use author_id as a foreign key in Book. Include id and name fields for Author, and id and title fields for Book.

Flask

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

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

# Define Author and Book models with lazy loading relationship here

Need a hint?

Use db.relationship without specifying lazy to get lazy loading by default.

2

Add a configuration variable to switch loading strategy

Create a variable called use_eager_loading and set it to False. This will control whether the relationship uses lazy loading or eager loading.

Flask

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

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

class Author(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50), nullable=False)
    books = db.relationship('Book', back_populates='author')

class Book(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(100), nullable=False)
    author_id = db.Column(db.Integer, db.ForeignKey('author.id'))
    author = db.relationship('Author', back_populates='books')

# Your code here

Need a hint?

Just create a simple variable use_eager_loading and set it to False.

3

Apply eager loading to the relationship based on the config variable

Modify the books relationship in the Author model to use eager loading with lazy='joined' if use_eager_loading is True. Otherwise, keep lazy loading (default). Use a conditional expression to set the lazy parameter.

Flask

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

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

use_eager_loading = False

class Author(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50), nullable=False)
    books = db.relationship('Book', back_populates='author', lazy='joined' if use_eager_loading else 'select')

class Book(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(100), nullable=False)
    author_id = db.Column(db.Integer, db.ForeignKey('author.id'))
    author = db.relationship('Author', back_populates='books')

# Your code here

Need a hint?

Use a conditional expression inside lazy= to switch between 'joined' (eager) and 'select' (lazy).

4

Query authors and access books to complete the example

Write code to create the database tables, add one Author named 'Jane Austen' with two Book entries titled 'Pride and Prejudice' and 'Sense and Sensibility'. Then query all authors using Author.query.all() and access the books attribute for each author to trigger loading.

Flask

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

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

use_eager_loading = False

class Author(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50), nullable=False)
    books = db.relationship('Book', back_populates='author', lazy='joined' if use_eager_loading else 'select')

class Book(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(100), nullable=False)
    author_id = db.Column(db.Integer, db.ForeignKey('author.id'))
    author = db.relationship('Author', back_populates='books')

# Your code here

Need a hint?

Use db.create_all() to create tables. Add author and books, commit, then query authors and access books.