Relationships (one-to-many) in Flask - Mini Project: Build & Apply

Flask One-to-Many Relationship Setup

📖 Scenario: You are building a simple blog application where each author can write many posts. You need to set up the data models to represent this one-to-many relationship using Flask and SQLAlchemy.

🎯 Goal: Create two Flask SQLAlchemy models named Author and Post where one author can have many posts. You will define the data structure, configure the relationship, and complete the model setup.

📋 What You'll Learn

Create an Author model with id and name fields

Create a Post model with id, title, and author_id fields

Set up a one-to-many relationship from Author to Post using SQLAlchemy

Use Flask-SQLAlchemy conventions and syntax

💡 Why This Matters

🌍 Real World

One-to-many relationships are common in web apps, like authors and their posts, customers and their orders, or teachers and their students.

💼 Career

Understanding how to model and implement relationships in databases is essential for backend development and working with ORMs like SQLAlchemy.

1

Create the Author model

Create a Flask SQLAlchemy model called Author with an integer primary key id and a string field name of length 50.

Flask

from flask_sqlalchemy import SQLAlchemy

db = SQLAlchemy()

# Define the Author model with id and name fields
# Your code here

Need a hint?

Use db.Column to define fields. The id should be an integer primary key. The name should be a string with max length 50.

2

Create the Post model with author_id

Create a Flask SQLAlchemy model called Post with an integer primary key id, a string field title of length 100, and an integer foreign key field author_id referencing author.id.

Flask

from flask_sqlalchemy import SQLAlchemy

db = SQLAlchemy()

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

# Define the Post model with id, title, and author_id fields
# Your code here

Need a hint?

Use db.ForeignKey('author.id') to link author_id to the Author model's id.

3

Add relationship to Author model

Add a relationship property called posts to the Author model that links to the Post model using db.relationship with backref='author' and lazy=True.

Flask

from flask_sqlalchemy import SQLAlchemy

db = SQLAlchemy()

class Author(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))
    # Add posts relationship here
    # Your code here

class Post(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(100))
    author_id = db.Column(db.Integer, db.ForeignKey('author.id'))

Need a hint?

Use db.relationship('Post', backref='author', lazy=True) inside the Author model.

4

Complete the model setup

Add the __repr__ method to both Author and Post models to return a string showing the name for Author and the title for Post.

Flask

from flask_sqlalchemy import SQLAlchemy

db = SQLAlchemy()

class Author(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(50))
    posts = db.relationship('Post', backref='author', lazy=True)

    # Add __repr__ method here
    # Your code here

class Post(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(100))
    author_id = db.Column(db.Integer, db.ForeignKey('author.id'))

    # Add __repr__ method here
    # Your code here

Need a hint?

Define __repr__ methods that return formatted strings with the model's main identifying field.